What does 1 Corinthians 14:15 mean?

Those with the spiritual gift of tongues should not exercise that gift in the church service without someone available to interpret (1 Corinthians 14:6–14). If nobody knows what is being said, nobody will benefit from hearing the speaker. That's why those with the ability to speak in tongues should ask God to give them the gift of interpretation, as well. Otherwise, they will be limited to using their gift of tongues only in prayer to God, who will understand what they are saying even if they do not.

Paul is not saying that praying to God in such a way is worthless, only that it doesn't involve the mind. That person's personal spirit alone is involved in speaking with God.

In Romans 8:26–27, Paul pictures prayer as communication between the spirit of a person and God, by way of God's Holy Spirit. In that passage, he shows that words are not always necessary. Here, though, he clearly expresses that being mentally engaged in prayer is better. This requires a person to understand what he or she is saying. Otherwise, the mind will not be involved.

Paul declares that he will not pray without the use of his mind. He will pray to God with both his own spirit and his mind. Singing praise songs is another kind of prayer. They are directed to God. Paul declares that he will sing praise to God with both spirit and mind, as well.

In order to accomplish this, Paul will only pray and sing with words he understands. If in a church service, he will only pray in tongues if he—or someone else—is able to interpret what is being said. Otherwise, the mind will be left out of the process.