Charismatic christians take the expression to refer to glosso-glossalalia. That is, to speaking a language not known to the speaker or others. Someone else may be given the gift to interpret it.
Other christians take the biblical references to refer to xeno-glossalalia. That is, to speaking a different but known human language. The references to 'interpreting' the tongue are about someone who can translate the language.
Pentecostal believers often argue that speaking in tongues is essential to full salvation. At the very least, they see it is a sign of the full blesssing of the holy Spirit.
The phenomenon of speaking in tongues is real enough, but I think there is plenty of evidence to suggest it is not divinely inspired but rather the product of a particular state of mind at the time people do it. I regard it as gibberish.

As with fighting Fire with fire, perhaps one fights gibberish [from the preacher] with Gibberish that needs no effort at composing, except perhaps to give it a rhythm similar to that of a language

Dont we all use our tongue to speak ? Smartass answer. Who can understand the language of tongues, and if nobody can understand it, doesn't it defeat the purpose of the communication?

A friend of mine who was a devout Pentacostal (and has since become a gleeful agnostic,) told me an interesting story about speaking in tongues.

Groups in her church would PRACTICE making repetitive sounds, in a singsong manner, punctuated with nonsense syllables to imitate speech. They had convinced themselves that this was only to "facilitate" god's language.

To be clear: they were not in a trance or any sort of altered state when they were practicing. The practice was done quite consciously, to make the "tongues" come more easily.

When we were growing up my two sisters started speaking gibberish to one another. They were pretending it was a foreign language and that they understood one another. Is that what you mean?

99% of the time is is gibberish.
My mother claimed to speak in tongues and it was just rubbish, jabbering noises, half words and moaning. She claimed it was "Adamic" the language of Edan and the angles and that anyone with enough faith in gawd could understand it.
I have accompanied her to (Charismatic and Toronto blessing) church on odd occasions as a boy where these building were filled with (mostly women) babbling away and mostly not understanding a word each other said.
I think personally it was even worse than when she was a salvationist singing Christian nonsense to Ricki Martin tunes.