This word is an indication of the musical performance of the psalm. The exact meaning of the word is not known. Some say it is a stop sign: the music has to stop or hold for a while for an interlude. That is just to emphasize what has just been sung. Others say it means forte,
which means that you have to play or sing with emphasis. Whether selah is a stop sign or a forte sign, it emphasizes the words that have just been sung. The use of selah here gives accents through which you can see the structure of the psalm:
Psalm 3:1–2 David’s pain – selah
Psalm 3:3–4 David’s trust – selah
Psalm 3:5–8 David’s victory - selah
2 many are saying of my soul, there is no salvation for him in God. Selah