When we pray in tongues, the Holy Spirit speaks directly to God through us, even if we ourselves don't understand what we are saying.

One idea is that tongues is a mode of utterance that can be understood by anyone regardless of his native language. St. Paul lists speaking in tongues as a gift of the Holy Spirit. There are three references in the Acts of the Apostles to speaking in tongues (Acts 2:4,6, 10:46 and 19:6). The object or destination of speaking in tongues is the community. The fact that the gift of tongues is not recorded in later books of the New Testament suggest that the gift may have even been ceasing during the biblical period. While not common in many Catholic churches, the charismatic gifts are recognized by the Church. Catholics are able to speak in tongues if they have been given the gift of tongues by the Holy Spirit. These gifts are referred to in 1 Corinthians 12:27-30. The phenomena of speaking or praying in tongues are among the charismatic gifts of the Holy Spirit. Speaking in tongues is a joint effort between you and the Holy Spirit. For most Christians, the gift of tongues is primarily a gift for prayer.

"Speaking or praying in tongues: The object or destination of praying in tongues is God himself, being the attitude of a person absorbed in a particular conversation with God. The first was in the very early Church, as recorded in the New Testament. He will supply the new language but He will not force you to speak. Speaking in tongues (also known as “glossolalia,” from the Greek word “glossa” meaning tongue or language) has been part of Catholic experience at two periods of our history. When you open your thoughts and you hear words in your mind or feel an impulse to speak in your throat or lips, give voice to whatever is there. Find out the Church teaching here. “Tongues” is a kind of prayer that can either be vocalized or internal and it’s when a person is able to pray in a language that they do not know. Another is that tongues are a “private prayer language” that is uniquely created by the Holy Spirit for each tongues-speaker. So why does it seem to be unpopular among Catholics? Augustine explained that this was because the Catholic Church now spoke the language of the nations, and tongue-speaking was only for purposes of evangelization (Aquinas agreed).

It can be used for private prayer or together with others who are also praying in this way. Neither idea is correct, and both stem from a failure to appreciate what the word “tongues” means.