You need to provide more context. I found the original article and in this case it refers to cops being cautious, metaphorically leaning back so that they're resting on the heels, because they're concerned for their own safety. In English we also have the phrase "to be on one's toes", meaning to be alert and ready for action.

As Patrick has said: following a person very closely (so close that the follower could literally be on the heels of the person being followed!).