The phenomenon known as Speaking in Tongues occurs mostly among those with a Pentecostal and Charismatic background. It is manifested when individuals, either in their private prayers or in a public meeting, say what sounds like an unknown language.
Some Christians believe so strongly in the manifestation of tongues speaking that those who display the ability are viewed as being more spiritually mature than others. Those who do not manifest this gift can be viewed as not converted at all.
Scriptural Justification
The primary scripture used to justify speaking in tongues is Acts 2:6. On God's annual holy day known as Pentecost, just ten days after Jesus ascended to heaven, the disciples gathered together in Jerusalem when they first received the empowerment of the Holy Spirit.
And suddenly there came from heaven a sound like the rushing of a powerful wind, and filled the whole house star where they were sitting.
And there appeared to them divided tongues as of fire, and sat upon each one of them. And they were all filled with the Holy Spirit; and they began to speak with other languages, as the Spirit gave them the words to proclaim.
Now there were many Jews who were sojourning in Jerusalem, devout men from every nation under heaven. And when word of this went out, the multitude came together and were confounded, because each one heard them speaking in his own language. (Acts 2:2 - 6, HBFV).
The most noticeable feature of this outpowering of power was that each of the disciples spoke in known languages (other than their own), and those in the audience, who were from many other nations, were surprised to hear them teach in the language of their country.
If "tongues speaking" is done privately, like in prayer, it is considered a "prayer language." Those who manifest this behavior may not even know what they are saying. Those who utter tongues in public, during a religious gathering, are believed by some to be delivering a message from God. The expectation is that alone with the message God will inspire to interpret it as well.
Those speaking in tongues usually do not claim that what they are saying is in a known human language. They believe they are uttering words in an unknown spiritual language (possibly an angelic language). Any interpretation offered is not seen as a literal but as a loose explanation of the message given by God's spirit.