Charles H. Spurgeon on Psalms 85:2

Selah. Rabbi Kimchi regards it as a sign to elevate the voice. The authors of the Septuagint translation appear to have regarded it as a musical or rythmical note. Herder regarded it as indicating a change of note; Mathewson as a musical note, equivalent, perhaps, to the word repeat. According to Luther and others, it means silence. Gesenius explains it to mean, "Let the instruments play and the singers stop." Wocher regards it as equivalent to sursum corda —up, my soul! Sommer, after examining all the seventy four passages in which the word occurs, recognises in every case "an actual appeal or summons to Jehovah." They are calls for aid and prayers to be heard, expressed either with entire directness, or if not in the imperative, "Hear, Jehovah!" or Awake, Jehovah! and the like, still earnest addresses to God that he would remember and hear, &c. The word itself he regards as indicating a blast of the trumpets by the priests. Selah, itself, he thinks an abridged expression, used for Higgaion Selah —Higgaion indicating the sound of the stringed instruments and Selah a vigorous blast of trumpets. From the "Bibliotheca Sacra, "quoted by Plumer.