The chlorine relationship is:

Free Chlorine (FC) is chlorine which is available to sanitize the water.

Combined Chlorine (CC) is chlorine which has combined with something else, usually nitrogen, during sanitization.

Together, they comprise Total Chlorine (TC). Simply put, FC + CC = TC.

CC doesn't have any sanitizing power, and is removed/converted/cleaned up by shocking the pool to a fairly high chlorine level, depending upon your CYA level.

Ideally, you want zero CC, and sufficient FC to maintain a clean, sanitary pool.