What does 1 Corinthians 14:3 mean?

Paul is comparing the relative values of the gifts of prophecy and of speaking in tongues. It may be that an excessive emphasis on tongues—and those with that gift—had created either division or an unbalance among the Christians in Corinth. Paul has urged them to desire the gift of prophecy more than the gift of tongues.

Now he reveals that the gift of prophecy serves a specific purpose in the life of the church. God means for this gift to be used to build others up: to encourage and comfort them. In other words, God would reveal some special truth, idea, or knowledge to a believer with this gift. That person was meant to speak this revelation to others in order to bring healing and encouragement. From this, we see that "prophetic" gifts were not always intended to reveal otherwise unknowable information from God.

Paul writes something similar about the public speaking gifts in Ephesians 4:11–12: "And he gave the apostles, the prophets, the evangelists, the shepherds and teachers, to equip the saints for the work of ministry, for building up the body of Christ."