Selah - from the Treasury of David

There is much more written about this word, but this was the part I found more interesting. The four musical points are quite fascinating, though perhaps some of them are speculative:

Verse 5. Selah. A little word, yet of no small difficulty to explain. Left out of the Bible by the vulgar translators, as though it were impertinent, where, let them consider, whether they come not within the verge of that malediction in Revelation 22:19 . The ancient interpreters did not much meddle with it, and our editions leave it without interpretation. But seeing "whatsoever things were written aforetime were written for our learning, that we through patience and comfort of the Scriptures might have hope" Romans 15:4 , and till "heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled" Matthew 5:18 , we have sufficient warrant after the example of the learned, and encouragement to make enquiry after the mind of the Holy Spirit, in that which he hath both commanded to be written, and hath commanded unto us. Wherein, like the crystal glass, I will rather present you with the true visage of antiquity, than use any newly framed feature or painting of my own.

Selah is mentioned seventy-four times in the Scripture, whereof seventy-one in the book of Psalms, and thrice in the prophet Habakkuk, which is written psalm wise; and it is ever placed in the end of a Psalm or verse, four places only excepted, where, like the sun in the midst of the planets, it is seated to conjoin the precedent words with the subsequent, and communicate splendour unto both. There was a threefold use of it in ancient times, whereof the first concerned the music; the second, the matter handled unto which it was affixed; and the third, the men or congregation assembled in the temple of the Lord, which two last may still have place among us Christians, who are ingrafted into the stock Christ, from whence the Jews were cut off, but from the first we cannot properly suck such nourishment as once they did.

First of the music. The king's choir ( 1 Chronicles 25:1-6 Psalms 62:1-12 , Epigrafh; 1 Chronicles 16:41 ) learned five things by it:

First. To make a little pause, stop, or stay, when they came to Selah, and to meditate awhile upon the matter foregoing.

Second. They knew by that cessation and interval that King David as he was prophesying unto the people, and praising God upon the loud sounding cymbals, was at that instant inspired and taught some new lesson. Wherefore, as men being in serious discourse, when they hear a sudden noise hold their peace to listen, saying, hark! see, lo! so David's heart being smitten by the voice of God's Spirit, the music ceased, stopped, and he checked himself as it were thus: "Speak, Lord, for thy servant heareth."

Third. It signifieth the change and variation of the music in some strains, or of the metre, or sense, or disjunction of the rhyme, or ceasing of some one sort of music, which howsoever St. Hierome makes some scruple of. The Septuagint, as often as they meet with Selah in the Hebrew text, in their Greek version translated it, the change of the song.

Fourth. It directed them to sing the same verse over again whereunto Selah was annexed. Lastly, it was their instruction to elevate and lift up their voices, praising God with louder voices and loud sounding cymbals. Selah called upon them for louder strains of music and shrillness of voice. But seeing the Jewish harmony and sweet melody is overwhelmed in the ruins of their glorious temple, we remain unskilled in their notes, which doth obscure our annotations upon it. Let this suffice for the "music."  - Edmund Layfielde.