Speaking in tongues is astonishing and fascinating ― a language we have never learned, or the language of “angels” (1 Cor 13:1).
The charism of tongues is primarily a gift for prayer ― the Holy Spirit speaks directly to God through us, even if we don’t understand our speech. “One who speaks in a tongue speaks not to men but to God” (1 Cor 14:2). The charism of tongues is a ‘gateway’ to other gifts and life in the Spirit.
St. Thomas Aquinas considered the charism of tongues paramount to evangelization. For example, one friend at university was enabled by the Holy Spirit to speak to a group of French students, even though he knew no French. He was able to help them and to share his faith with them. The charism of tongues signals the presence of the Holy Spirit, though one may receive the Holy Spirit without speaking in tongues.
The most obvious example of speaking in tongues happened at the very first Pentecost (Acts 2:4-11), when the disciples shared the gospel and the people heard “in their own tongues the mighty works of God!” In public worship, a person may prophesy in tongues, and another person may interpret the speech for the group to hear God’s word. The interpretation of tongues is another charism.