What does 1 Corinthians 14:17 mean?

Verse 12 indicated that the Christians in Corinth should be striving at building up the church by building up each other. That's the reason spiritual gifts are given by God. That's what those gifts are intended to be used for. If spiritual gifts don't help with that purpose, they should not be displayed in the church.

That's why Paul is restricting the exercise of the gift of speaking in unknown languages during church services if no interpretation is available. Some of the Corinthians may have objected that they were using the gift to pray to God, and He understood them. Paul's response is that praying to God without knowing what is being said makes the experience only a spiritual one and not mentally engaging. For those who listen to unintelligible words, it is neither spiritual nor intellectual. It is meaningless.

In the previous verse, he wrote that those who hear a prayer of thanksgiving to God without knowing what is being said cannot respond by saying "Amen." Apparently, this was a common response in the church to prayers of blessing or giving thanks to God. Saying "Amen" was a way of expressing agreement with the one who was praying. This practice continues in many churches today.

Paul writes now that the problem is not with the prayer itself. The speaker may have given thanks "well enough," but they have not contributed to the mission of the church: They have not built anyone up.

Setting the discussion of spiritual gifts aside, Paul's teaching here shows that one purpose of public prayer in church is to build up those who hear, as well as communicating to God personally.