סלה is pronounced “Selah” in English. Appearing 71 times in the Psalms, the word remains a mystery to modern day translators. There are many theories, but nobody knows for sure what it means.
Selah is probably either a musical mark or an instruction on the reading of the text, something like “stop and listen.”
The Psalms were mostly designed to be sung or accompanied by musical instruments. Many of the Psalms that mention a “choir master” also include “Selah.” This suggests a strong musical link with the word.
Some scholars have suggested that “Selah” is used to separate stanzas or distinct thoughts. It could serve to indicate a musical rest or pause. But this explanation doesn’t work in all instances. The word does seem to interrupt the flow of a logical strain of prose in some places. It also occurs at the end of a psalm and provides contrast between various thoughts within a psalm.
We just don’t know for sure what the word means, and we have no real good way to test various hypothesises. I kind of like that this word is a mystery. This fact invites personal interpretation and sparks the imagination of the reader. For some it may be a musical rest or an invitation to pause and ponder the greatness of God. For others, it could just be a mystery. This points to one of the great realities of God – He is beyond comprehension, and we encounter God by stopping to ponder His greatness.