Candor usually means the quality of being open, honest, and sincere. If someone tells you they think you are dumb, you might reply with, "While I appreciate your candor, I don't think we need to be friends anymore."
The corresponding adjective is candid, as in "I want you to be candid. Do these pants make me look fat?" Candor is from French candeur, from Latin candor, from candēre "to shine, be white." The color white is associated with purity, which is a now obsolete meaning of candor.