Glossolalia, or speaking in tongues, is the practice of some religious people of uttering unintelligible, often meaningless, speech-like sounds, usually during periods of intense religious worship.
The practice is most common among evangelical fundamentalist Christians, who consider it an indicator of a person having been "baptized in Holy Spirit." The practice among Christians is derived from the supposed xenoglossy1 in the Bible books of Acts and 1 Corinthians, where the apostles are said to have spoken in tongues in immediate response to the Holy Spirit acting upon them.
Notably, modern glossolalia does not mirror the practice as stated in the Bible, wherein listeners of various languages are said to have understood the apostles as if they were being spoken to in their own language. (Acts 2:7-8) Rather, the sounds uttered by modern Christians when "speaking in tongues" are not understood by anyone.
The practice among Christians was revived in modern times by preachers such as Kenneth Hagin and Kenneth Copeland in the mid-seventies as part of the "Pentecostal revival." Some, like Hagin, took the practice a step further by engaging in even stranger acts "under the influence of the Spirit." At one sermon, Mr. Hagin was speaking of being "drunk in the Spirit", when he suddenly started acting as though he were staggering drunk- laughing, slurring his words, and stumbling around the room, an action that the audience members immediately started mimicking. On other occasions, Mr. Hagin began hissing like a snake, after which the audience of course immediately started to also make animal sounds, with some lowing like cows, some barking, and some squealing like pigs.
Glossolalia is often described as a "prayer language" or "angelic language." It is said by many evangelicals that glossolalia can be used to pray when you're not quite sure what to pray, or when you wish to speak to God privately- ignoring the fact that if God is omniscient, he doesn't need prayer at all, let alone prayer in a "language" that even the believer does not understand. Some say that it is a language that Satan cannot understand. It is also interesting to note that if it is, indeed, an "angelic language", then angels are so advanced as to have no need of such complex languages as humans use, since most glossolalia seems to consist of the same half-dozen or so syllables repeated over and over again. "SH" and "LA" sounds seem to make up a majority of this "speech", with many extensively rolled "R"s thrown in.
1. Both glossolalia and xenoglossy include the knowledge of or ability to speak a language one does not understand, though glossolalia is more often used to describe the unintelligible uttering practiced by Christians. Strictly speaking, however, neither term precisely describes being understood simultaneously in different languages.