A metrical psalter is a special book that helps people sing songs about God called psalms. These psalms were written a long, long time ago in a language called Hebrew. But the problem was that not everyone in the churches understood Hebrew, so they couldn't sing the songs together.
To fix this problem, some people translated the psalms into different languages and also put them into poetry, with words that rhyme and have a beat. This made it much easier for everyone to sing together, and they called this book a metrical psalter.
The metrical psalter has many different psalms, each with their own special tune, called a hymn. When people go to church, they can open the metrical psalter and follow along with the words and sing with the music. It's like singing along to your favorite song, but about God!
So, a metrical psalter is a special book that helps people sing together in church by translating the old psalms into easier-to-understand and sing poetry.