This apparently means that the speaker is stepping towards the man, but haltingly - the man is apparently someone to beware of. And when the man holds up his hand in a "stop where you are" gesture, this is enough to cause the speaker to stop in his tracks, stop so suddenly, in fact, that he "rocks back on his heels": sways backwards while standing still, balanced on the heels of his feet only. Note that this phrase is often used in a figurative sense: one might not actually be balanced on his heels as described above (this is a very awkward physical stance that rarely occurs in real life), but rather, has stopped himself suddenly from proceeding, either literally or figuratively.
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This apparently means that the speaker is stepping towards the man, but haltingly - the man is apparently someone to beware of. And when the man holds up his hand in a "stop where you are" gesture, this is enough to cause the speaker to stop in his tracks, stop so suddenly, in fact, that he "rocks back on his heels": sways backwards while standing still, balanced on the heels of his feet onl