Exclusive Psalmody means that in some Christian church services, they only sing psalms (songs from the Book of Psalms in the Bible) and they don't sing any other kind of songs or music. It's kind of like a rule they have.
So, instead of singing popular songs or hymns, they only sing songs from the Book of Psalms. This is because they believe that the Bible only tells us to sing the psalms and not any other kind of songs or music.
Some people really like singing the psalms because they think it's a good way to praise and worship God. But other people don't like it because it can sometimes be a bit repetitive and they may want to sing other songs that they like.
Overall, exclusive psalmody is a way for some Christians to worship and praise God by only singing songs from the Bible's Book of Psalms.