Speaking in tongues is mainly practiced by Pentecostal churches but is also taught by some groups in the Catholic charismatic movement. It is one of the seven gifts of the Holy Spirit and its introduction to early Christians is narrated in Acts 2:1–4 on a day called Pentecost.
Speaking in tongues is thus described technically as the fluid vocalization of speech that lacks comprehended meaning. The Holy Spirit supposedly fills a person which commonly takes place during worship and the host suddenly blurts out unintelligible utterances that are sometimes accompanied by uncontrollable bodily convulsions.
But some verses suggest the sparing use of this gift. Paul tells about its meaninglessness if not complemented with revelation, prophecy or teaching (1 Corinthians 14:6). He also admonishes practitioners to pray that they understand what their mouths speak of (1 Corinthians 14:3).
In such instances, Paul mentions that the presence of an interpreter is required otherwise, "let each of them keep silent in church and speak to himself and to God" (1 Corinthians 14:27–28). These instructions are not commonly observed. A cacophony of wailing voices will burst out from worshippers at church, but nobody interprets nor bothers to ask the tongue speakers if they understood what they were babbling about.
This is why not all Evangelical churches observe this gift in deference to Paul's advice that "I would rather speak five intelligible words to instruct others than 10,000 words in a tongue" (1 Corinthians 14:9). There are also suggestions that this gift is manifested only on rare occasions and the outbursts experienced by practitioners are usually emotional in nature and not spiritual.
Since the aforementioned instructions accompanying speaking in tongues are mostly unheeded, the best way to determine the validity of its expression is by the fruit produced by the tongue speaker (Matthew 7:16). True holiness is manifested not only during worship but on how we live our faith in our daily lives.