This word, which is found only in the poetical books of the Old Testament, occurs seventy-one times in the Psalms and three times in Habakkuk. It is probably a term which had a meaning in the musical nomenclature of the Hebrews, though what that meaning may have been is now a matter of pure conjecture. (Gesenius and Ewald and others think it has much the same meaning as our interlude,-a pause in the voices singing, while the instruments perform alone.)
SELAH
Meaning of SELAH in English
Smith's Bible English dictionary. Английский библейский словарь Смита. 2012