we count from 1 to 9 then 10.
and we start using 2 digits to at 10, because we are counting in "base 10"
We only have 10 numbers to work with, 0 through 9
Okay... that's what we're all familiar with
Now with "binary" we are using "base 2"
Meaning that we can only work with 2 numbers, 0 and 1
and this means that as soon as we get to "2" when we count - we start using 2 digits, in other words, our only numbers are 0 and 1 and we can never say "2", instead we have to say 10 (when we mean 2).
3 would 11 (because it is one higher than 2, which is "10")
4 would be 100 and 5 is 101 (because it is one higher than 4.
You might ask, "Huh? Why all the trouble?"
Well, it's when we start using computers that "binary" code really gets usefull.
Because they can understand "on, off" they are pretty well adapted to "0=off, 1=on" and binary code.
Here's a tutorial about binary code and computers
http://jamalifouru.com/video-learn-learning-videos/learn-computer-Tutorial/index.php?id=3