Psalm 151 is an additional Psalm, not found in most Bibles. Ascribed to David the text praises God for selecting him to become king and allowing him to defeat the Philistine warrior, likely Goliath, (see 1 Samuel 17 for the complete story).
Psalm 151 is an Apocrypha book and not included in all versions of the Bible. The Revised Standard Version (RSV), Common English Bible (CEB), Eastern Orthodox, and Ethiopian Bibles all include Psalm 151. The Septuagint, an ancient Greek translation of the Jewish scriptures, which was widely used in Jesus’s day, also contains Psalm 151.
For more information, see why “Christians Should Consider the Entire Bible.”