What is the Biblical gift of tongues?

 I once heard a young lady say that she had been taught how to speak in tongues but was embarrassed by the sound of her "tongues." Our small group leader asked her to speak her "new language," and she hesitatingly did. It was a one syllable sound repeated over and over again. It was "dadadadadadadada," said very quickly. The leader and others told her it was the gift of tongues. And everyone went home convinced that this young lady had the genuine gift of tongues.

Dear reader, that was not the Biblical gift of tongues no matter how one might spin it. It was gibberish foisted upon a novice by deceived people who thought (sincerely) that everyone is supposed to speak in tongues. A cursory reading of 1 Corinthians 12 and 14 would have told them otherwise. 

And that is an excellent segue into my next point, people caught up in the Charismatic Movement as well as those in the Pentecostal Movement and the Word of Faith Movement are given to taking Scripture out of context with no qualms about their abuse of the word of God. I am convinced that these deceived adherents of those movements would repent of their error if someone could reach them with the revelation that one must not take Scripture out of context. They are simply, if not willingly, ignorant.

The Biblical gift of tongues is an earthly language unlearned by the speaker; they are the work of the Spirit. None of the gifts of the Spirit can be taught as Sid Roth and many, many others teach their hearers. The word of God makes it undeniably clear that the Spirit gives the gifts as He wills, and they cannot be taught by man. This is what was manifest on the Day of Pentecost when the Spirit was poured out on the 120 disciples gathered in the upper room in Jerusalem.   

ESV  Acts 2:1 When the day of Pentecost arrived, they were all together in one place.

 2 And suddenly there came from heaven a sound like a mighty rushing wind, and it filled the entire house where they were sitting.

 3 And divided tongues as of fire appeared to them and rested on each one of them.

 4 And they were all filled with the Holy Spirit and began to speak in other tongues as the Spirit gave them utterance.

 5 Now there were dwelling in Jerusalem Jews, devout men from every nation under heaven.

 6 And at this sound the multitude came together, and they were bewildered, because each one was hearing them speak in his own language. (Acts 2:1-6 ESV)

The Greek text of that passage uses the word διαλέκτῳ  in v.6 translated "language." And the word is our source for the English word "dialect." Here's Webster's meaning of dialect: Definition of dialect

1linguistics

a: a regional variety of language distinguished by features of vocabulary, grammar, and pronunciation from other regional varieties and constituting together with them a single languagethe Doric dialect of ancient Greeka dialect of Chinese spoken in Hong Kong

b: one of two or more cognate (see COGNATE entry 1 sense 3a) languagesFrench and Italian are Romance dialects

c: a variety of a language used by the members of a groupsuch dialects as politics and advertising— Philip Howard

d: a variety of language whose identity is fixed by a factor other than geography (such as social class)

The Jews from various geographical areas all heard the baptized-in-the-Holy Spirit disciples speaking in their dialect, dialect being synonymous with language. The Greek text has this which is translated as "other tongues:" ἑτέραις γλώσσαις "other tongues," languages (Acts 2:4 BGT)4 And they were all filled with the Holy Spirit and began to speak in other tongues as the Spirit gave them utterance. (Acts 2:4 ESV) In the context it is unmistakably referring to languages known by the hearers yet languages the disciples had not learned.

The event of the Day of Pentecost defined once and for all the meaning and application of the gift of speaking in tongues. When one reads Paul's instructions and information concerning tongues, he refers to a known earthly language unlearned by the speaker in a local church setting. That includes the verse that some have designated a "private prayer language."  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. (1 Cor. 14:2 ESV) 9 So with yourselves, if with your tongue you utter speech that is not intelligible, how will anyone know what is said? For you will be speaking into the air. (1 Cor. 14:9 ESV)

14 For if I pray in a tongue, my spirit prays but my mind is unfruitful. (1 Cor. 14:14 ESV)

 21 In the Law it is written, "By people of strange tongues and by the lips of foreigners will I speak to this people, and even then they will not listen to me, says the Lord."

 22 Thus tongues are a sign not for believers but for unbelievers,  (1 Cor. 14:21-22 ESV)

27 If any speak in a tongue, let there be only two or at most three, and each in turn, and let someone interpret.

 28 But if there is no one to interpret, let each of them keep silent in church and speak to himself and to God. (1 Cor. 14:27-28 ESV)

When one with the Biblical gift of tongues, not falsely taught them by sometimes well-meaning church folk, knows there is no one in his/her local church to interpret, then that one is instructed to speak in such a way that no one can hear them. This is described by Charismatics and Pentecostals as a prayer language. I can see that. But tongues spoken quietly in a local church setting does not mean that speaking in tongues has morphed into gibberish. Tongues is always an earthly language unlearned that must be interpreted. Otherwise, the speaker is to speak to himself and to God.