Sela appears in the bible 80 times, 74 times in Psalms and 6 times in Habbakuk.
It means 'rest'. It more a pause in the reading of a song of praise (i.e. 'Psalms' ) than having any other meaning.
If you are singing a Psalm, just pause for a moment at the 'Sela' and you should be alright. It is there to allow for a moment's reflection on what has just been said.
The first occurences are in Psalm 3, at the end of certain verses. The word 'Sela' has also been described as a musical interlude.
The word sela itself is Hebrew, being a transliteration of the Hebrew letters appearing in bible manuscripts at those points in the ancient texts.