Behab stands for Monday, Thursday, Monday — a way to seek forgiveness if there was any improper behavior during Succos, especially with Chol Hamoed, and to daven for the crops as the seasons change.

These days were meant as fast days, but nowadays, we usually just say selichos instead. You can say selichos at home, but without the 13 Middos. Elya Rabba 492:2,3.