In plain words
One of the 150 prayers and songs in the Bible’s book of Psalms.
The fuller definition
One of the 150 prayers and songs in the Bible’s book of Psalms, the prayer book of Israel and of Jesus. They include praise, thanks, anger, grief and despair. One is sung or said in most Lutheran services.
Where it comes from
Psalm 23
In The Bible: Its books, its translations, and how Lutherans read it.