Paul wants us to understand the excellence of prophecy for both believers and unbelievers. Prophecy is edifying precisely because it can be understood by all. In order for tongues to be edifying for others they need to be interpreted (14:5). In fact, tongues will scare away unbelievers (14:23). Paul quotes from Isaiah 28:11 to show that unknown tongues don’t engineer belief (14:21). If you think that verse 22 is confusing, you are not alone! It could be understood as a Corinthian statement that Paul contradicts in verse 23. It could also be understood the way an ancient commentator paraphrased it: “Prophecy makes believers of unbelievers; speaking in tongues leaves the unbeliever to himself.” Paul repeats his main point in verse 25: prophecy (teaching) is for use in corporate worship because it causes conviction and reveals that God is present (14:25).
There is some debate over what “tongues” actually are. Verses 10-11 and 21 seem to indicate that a person who is speaking in tongues is supernaturally empowered to speak an earthly language that they do not know (14:4). It’s edifying to them because they are experiencing a supernatural event. It’s not edifying to others because they don’t understand it without interpretation (14:5). Once again, we are reminded that gifts are for the edifying of the church body (14:12), which applies regardless of how you view tongues.