What exactly does "Speaking in tongues" mean anyway?

Glossolalia or speaking in tongues is the fluid vocalizing (or, less commonly, the writing) of speech-like syllables, often as part of religious practice. There are many documented cases around the world and all through history, especially throughout Christian history. The significance of Glossolalia has also varied with time and place, with some considering it a part of a holy language. Others believe that the vocalizations are meaningless, and cite hypnosis, mental illness, and social learning as scientific explanations.