What does selah mean (i.e amen selah)?
Selah Means forever. In Tehillim metzudas Zion writes כמו לעולם.
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.
Rav Hirsch explains it to mean that the foregoing thought should be held constantly in one’s mind. (Commentary to Tehilim 3, if I’m not mistaken).
Forever like a stone