A teacher once proposed a theory that it is a musical term used by the conductor for the Leviim’s song. Don’t remember if it was just a theory or there were any perushim to back it up.
In Tehilim 3:5 according to the Malbim it is a sign of a finishing touch of a topic. The Rid says that this indicated to hit a musical instrument. The Targum and Even Ezra says that it means forever.