What does 1 Corinthians 14:14 mean?

Not every believer was given the spiritual gift of being able to speak in unknown languages in Paul's day. Paul wrote that he wished all those in Corinth had this gift (1 Corinthians 14:5), but clearly that wasn't the case.

The Corinthian church seemed widely impressed with this gift. Paul has not discounted speaking in tongues, but he has corrected them about the practice of exercising the gift publicly, in the church service, if no interpretation was available.

In fact, Paul has written in the previous verse that those with the gift of tongues should ask God to give them the gift of interpretation of tongues, as well. This would allow their gift to be used in the church service to benefit everyone. It would also allow their own personal experience of praying to God in an unknown language by the power of the Holy Spirit to be more meaningful for them.

He writes in this verse that if he prays in a tongue, a language he does not know, his own personal spirit prays, but his mind does nothing. It is "unfruitful" because the one praying does not know what he is saying. Paul does not reject this as a meaningless experience, only a less ideal one. It should not take place in the church service, since nobody who hears the prayer will be able to engage with what is being prayed on either a mental or spiritual level.