It is speaking of a particular language other than your mother tongue or your native tongue. That is speaking in tongues. For example you are an American, then you speak in other tongues which is strange.
As it is recorded in the book of Acts, when the apostles who are all Galileans spoke in Galilean language and what they have spoken was by the power of the Holy Spirit, reached the ears of the listeners in their own native tongue
The Acts 2:7-
"And they were all amazed and marveled, saying one to another, Behold, are not all these which speak Galilaeans?"
The Acts 2:8-
"And how hear we every man in our own tongue, wherein we were born?"
But now the pastors and the members of religious denominations that espouse the belief of speaking in tongues now speak of things which nobody knows. So it is speaking of a particular language other than your mother tongue or native tongue.
But the preachers now or these groups now that claim to speak in tongue are speaking of things which you cannot find anywhere on earth because they are not really languages or dialects, they are invented expressions.
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