An RBG provides backtracking resistance relative to time T if it provides assurance that an adversary that has knowledge of the state of the RBG at some time(s) subsequent to time T (but incapable of performing work that matches the claimed security strength of the RBG) would be unable to distinguish between observations of ideal random bitstrings and (previously unseen) bitstrings that are output by the RBG at or prior to time T. In particular, an RBG whose design allows the adversary to "backtrack" from the initially-compromised RBG state(s) to obtain knowledge of prior RBG states and the corresponding outputs (including the RBG state and output at time T) would not provide backtracking resistance relative to time T. (Contrast with prediction resistance.)
Sources:
NIST SP 800-90A Rev. 1