"Selah" is a Hebrew word of uncertain meaning that appears 71 times in the Book of Psalms (and three times in Habakkuk), often understood as a musical or liturgical instruction to pause.
Psalms 3:2 in the King James Version reads:
Many there be which say of my soul, There is no help for him in God. Selah.
The word appears at the end of Psalm 3:2, one of its earliest occurrences in the Psalter. Read it in context at Psalms 3:2, or the full chapter at Psalms 3. Encyclopedia entry: en.wikipedia.org/wiki/Selah.