What is speaking in tongues?

[TonyChanYT]

Act 1:

3 They saw what seemed to be tongues of fire that separated and came to rest on each of them. 4 All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them.
5 Now there were staying in Jerusalem God-fearing Jews from every nation under heaven. 6 When they heard this sound, a crowd came together in bewilderment, because each one heard their own language being spoken. 7Utterly amazed, they asked: “Aren’t all these who are speaking Galileans? 8 Then how is it that each of us hears them in our native language?

This phenomenon is called xenolalia: A believer speaks in a human language that he doesn't know.

There is another kind of speaking in tongues, 1 Corinthians 14:

2, “For anyone who speaks in a tongue does not speak to people but God. Indeed, no one understands them; they utter mysteries by the Spirit.”

This latter kind of speaking in tongues is called glossolalia: A believer speaks in a non-human language.

What's the purpose of glossolalia?

4 Anyone who speaks in a tongue edifies themselves, but the one who prophesies edifies the church. 5I would like every one of you to speak in tongues, b but I would rather have you prophesy. The one who prophesies is greater than the one who speaks in tongues, unless someone interprets, so that the church may be edified.

1. It edifies the speaker himself.
2. When it is interpreted, it edifies the listeners in the church.

Is glossolalia available today?

I think so. Jesus promised in Mark 16:

17 And these signs will accompany those who believe: in my name they will cast out demons; they will speak in new tongues; 18 they will pick up serpents with their hands; and if they drink any deadly poison, it will not hurt them; they will lay their hands on the sick, and they will recover.”

Does every believer have this gift?

No, 1 Corinthians 12:

7 Now to each one the manifestation of the Spirit is given for the common good. 8 To one there is given through the Spirit a message of wisdom, to another a message of knowledge by means of the same Spirit, 9to another faith by the same Spirit, to another gifts of healing by that one Spirit, 10 to another miraculous powers, to another prophecy, to another distinguishing between spirits, to another speaking in different kinds of tongues, a and to still another the interpretation of tongues. 11All these are the work of one and the same Spirit, and he distributes them to each one, just as he determines.

So it is up to the Holy Spirit to hand out these gifts.

Glossolalia seems to be some kind of spiritual/angelic language, 1 Corinthians 13:

1 If I speak in the tongues of men and of angels, but have not love, I am a noisy gong or a clanging cymbal.

1 Corinthians 14:

2 For one who speaks in a tongue speaks not to men but to God; for no one understands him, but he utters mysteries in the Spirit.

Do you have to speak in tongues in order to be saved?

No, not every Christian is required to speak in tongues.