Boyd’s Bible Dictionary:

Place for herding sheep, especially at night
Usually built strong to keep out wild animals (Num. 32:16; 2 Sam. 7:8; John 10:16). The bold, cote, or enclosure was also the place where the sheep were collected at shearing time (Jer. 23:3; Zeph. 2:6), which was a season of festivity (1 Sam. 25:7-11; 2 Sam. 13:23). Hence “shearing-house” (2 Kings 10:12-14).

From Manners and Customs of the Bible:

John 10:1. He that entereth not by the door into the sheep-fold, but climbeth up some other way, the same is a thief and a robber.
In this beautiful figure reference is made to the place of shelter for the sheep where they might repose at night, and be safe from the attacks of wild beasts. The modern sheep-folds of Syria, which no doubt resemble those of ancient times, are low, fiat buildings opening into a court, which is surrounded by a stone wall, protected on the top by a layer of thorns. A doorway carefully guarded forms the entrance. Sheep-folds are referred to in a number of passages. See Numbers 32:16,24,36; 1 Samuel 24:3; 2 Chronicles 32:28; Psalm 78:70.