If you spend any time at all reading the Psalms, you will inevitably run across the Hebrew word "selah" scattered throughout the text. Here is a breakdown of how this word is used.
Occurrences of "Selah"
The Hebrew word "selah" occurs 71 times in 39 Psalms.
Selah also occurs 3 times in Habakkuk's song (ch. 3).
The Meaning of "Selah"
The main question that arises is, of course, what does it mean? There have been several proposals, most popular of which is something like "rest" or "ponder". This understanding views "selah" as a musical interlude. This view, however, is mostly a guess without much solid evidence.
The most convincing argument from the text of Scripture views "selah" as an acronym meaning "return to the above [or, to the beginning], O Singer".
Sub Lima'lāh Hashār
In this view, "selah" is a musical instruction similar to "D.C."(da capo), which means "from the head/beginning". When reading a psalm that uses "selah", it makes the most sense to take the first line (usually two) and use it as a refrain everywhere that "selah" occurs.
NOTE: This material has been summarized from "The Meaning and Use of Selah" in The Danville Review (June 1864)