What does fold mean in Bible?

Answer 1

A fold was an enclosure made of stone and topped with thorn bushes where a flock of sheep was placed at night for protection against predators.

The term fold was somewhat bastardized and cooped by Christianity, where a flock was used as an analogy for a congregation. So to "join the fold" was to join the flock in it's protected place, the church. Where they would be protected not from predators but from the evil of temptation and free will.

The flock or congregation takes protected shelter in the "fold" which is analogous to the physical church itself.

Answer 2

The fold in The Bible means the members of the church.