Two Types Of Tongues
Paul distinguishes between Tongues + Interpretation and Tongues as a Private Prayer Language.
A. Tongues + Interpretation
Tongues with interpretation= God speaking to unbelievers in a way that will catch their attention.
Example: DANIEL 5:1-30 The Writing on the Wall: a written warning to an arrogant king, interpreted for Belshazzar by the young Daniel.
B. Tongues as A Private Prayer Language
Tongues as a private prayer language= for believers to praise and worship God more freely.
In 1 CORINTHIANS 14:14-19 Paul says he prayed in tongues more than any of the Corinthian Christians as a way of praising God. But he also said he would rather they spoke in tongues in the meeting only if they also had the interpretation, so any unbelievers present could understand what was being said.
Speaking in tongues (or glossolalia=tongue-babble) is like being able to pray with one's brain in neutral and without the constraints of having to find words for what you want to say to God. It's like a fountain of words just bubbling up from inside you, filling you with praise to God for his wonderful love and being able to express your love for him without getting tongue-tied. Many Christians also find singing in tongues a wonderful way of praising God in song - something else Paul mentions he did.
Problems With Speaking In Tongues
Unfortunately, speaking in tongues has been portrayed as a sign of special status or maturity by some Christians over the centuries.
According to this view there are two levels of Christians - those who speak in tongues and those who don't. The problem is that speaking in tongues is a gift conferred on individuals as the Holy Spirit decides, Paul says in 1 CORINTHIANS 12:11. So we cannot claim any credit for the ability or otherwise, any more than someone gifted with the supernatural gift(s) of HEALING(s) can claim the credit for healings resulting from his/her exercise of a God-given gift.
Some Christians have claimed that to be a born again (ie real) Christian you must be able to speak in tongues.
In many cases their arguments are based on a failure to understand the preceding issues and also on an extrapolation of specific examples mentioned in The Book of ACTS to all believers.
It is true that one sign of genuine conversion in the Book of ACTS was speaking in tongues in praise and worship to God for the gift of salvation. But in the instances quoted tongues are used as a private praise & prayer language and were for the building up (or "edification") of the individuals involved, not for unbelievers. If they had been meant for unbelievers, there would have been an interpretation - which is absent in each instance.
A Word of Warning
Tongues speaking is no guarantee that a person is a Christian. People of other faiths and occult practitioners speak in (demonic) tongues, and even babies babble.