There are a few differing views on this. Some have applied it to the current generation in which they/we live, or some generation to come, in reference to the end times, the coming of Christ. However, the typical reformed biblical scholar view sees it in reference to the physical destruction of the temple.
We know the temple was destroyed in about 70AD. We also can know that Christ was crucified in about 33 AD, (Pontius Pilate was procurator of Judea from 26-36 AD) So this talk that Jesus is having as recorded in Mark 13 is taking placed about 33 AD. So, Jesus is making a prophetic statement about what will happen in 37-40 years.
Chapter 13 starts of;
13 As Jesus was leaving the temple, one of his disciples said to him, “Look, Teacher! What massive stones! What magnificent buildings!” 2 "Do you see all these great buildings?” replied Jesus. “Not one stone here will be left on another; every one will be thrown down.”
Later He is asked privately by Peter, James, John and Andrew when will this take place, and what are the signs.......and this is where we find the fork of understanding today, where some apply it one way and others another way.
Jesus lays out the sings for them; .....
“Watch out that no one deceives you. 6 Many will come in my name, claiming, ‘I am he,’ and will deceive many. 7 When you hear of wars and rumors of wars, do not be alarmed. Such things must happen, but the end is still to come. 8 Nation will rise against nation, and kingdom against kingdom. There will be earthquakes in various places, and famines. These are the beginning of birth pains.
9 “You must be on your guard. You will be handed over to the local councils and flogged in the synagogues. On account of me you will stand before governors and kings as witnesses to them. 10 And the gospel must first be preached to all nations. 11 Whenever you are arrested and brought to trial, do not worry beforehand about what to say. Just say whatever is given you at the time, for it is not you speaking, but the Holy Spirit.
12 “Brother will betray brother to death, and a father his child. Children will rebel against their parents and have them put to death. 13 Everyone will hate you because of me, but the one who stands firm to the end will be saved.
14 “When you see ‘the abomination that causes desolation’ standing where it does not belong—let the reader understand—then let those who are in Judea flee to the mountains. 15 Let no one on the housetop go down or enter the house to take anything out. 16 Let no one in the field go back to get their cloak. 17 How dreadful it will be in those days for pregnant women and nursing mothers! 18 Pray that this will not take place in winter, 19 because those will be days of distress unequaled from the beginning, when God created the world, until now—and never to be equaled again.
20 “If the Lord had not cut short those days, no one would survive. But for the sake of the elect, whom he has chosen, he has shortened them. 21 At that time if anyone says to you, ‘Look, here is the Messiah!’ or, ‘Look, there he is!’ do not believe it. 22 For false messiahs and false prophets will appear and perform signs and wonders to deceive, if possible, even the elect. 23 So be on your guard; I have told you everything ahead of time.
24 “But in those days, following that distress,
“‘the sun will be darkened,
and the moon will not give its light;
25 the stars will fall from the sky,
and the heavenly bodies will be shaken.’
26 “At that time people will see the Son of Man coming in clouds with great power and glory. 27 And he will send his angels and gather his elect from the four winds, from the ends of the earth to the ends of the heavens.
28 “Now learn this lesson from the fig tree: As soon as its twigs get tender and its leaves come out, you know that summer is near. 29 Even so, when you see these things happening, you know that it is near, right at the door. 30 Truly I tell you, this generation will certainly not pass away until all these things have happened. 31 Heaven and earth will pass away, but my words will never pass away.
The way I think you should study this is from all the views. Ask yourself whats a fair and reasonable way to view this? If we view it as the times in which Christ lived, then we need to reconcile all these signs within that generation. For most of it, that's not much of a problem; wars and rumors of wars, brother will betray brother and so on. We can apply that and other things to the 40 years leading up to the destruction of the temple in 70 AD, BUT...the sun will be darkened, the moon will not give it's light, the stars will fall from the sky? if we are going to apply that to that generation, then how do we reconcile these events? what does Jesus mean? could he be talking about something to happen later? That is, after 70AD? Something still ahead? Yes, He could be making a duel prophetic statement here.
So, I'd suggest you take what you do know, what you CAN know, and focus on that first. Then take what you don't know and what you can't know and reconcile that to the other.
CAUTION: It can be easy to go off the rails with this. In my opinion many have. That is why I'm suggesting to first stick with what you can know and what you do know. Like the destruction of the temple in 70 AD. We know this took place. We can know this. We know from historical records, from archeological record, there are eye witness accounts recorded. DO NOT make up or ASSUME what you don't know or CAN'T know. Instead leave that to be reconciled later, or apply several reasonable possibilities, and leave it be. otherwise, if you assume or make solid explanations on what you can't know, then it will simply be your own word.
Right now we have many people saying what is fact on what they can't know, namely the coming of Jesus Christ within this generation. They interpret scripture with the bible in one hand and the newspaper of the world in the other hand. They tie what they read in the news to the bible, and make all these predictions that so far everyone of them have been dead wrong. Some of these people may have been sincere, but it's also safe to assume some are purposely deceiving many for their own gain.
John Hagee's blood moons book is an example of one that comes to mind.
So, while I can't say much on the sign seekers of the end times, I can drop a few other resources on you for the reformed view in relation this text you're wondering about. You can read some of the views I agree with at these links. Hope this helps. Blessed study to you.
http://www.ligonier.org/learn/keywords/end-times/
http://www.gotquestions.org/search.php?zoom_query=end times&search.x=0&search.y=0
Here is another one, I sort of skimmed it. It's a little harsh but straightforward.
http://www.bible.ca/pre-mt24-this-generation.htm
, was not one of those things.