1. Literally, to attentively look where one is placing one's feet while walking, running, etc. This trail hasn't been maintained very well by the park, so mind your step.

2. By extension, to be wary of or cautious about what one does while in a tense, dangerous, or delicate situation. Though she's renowned for her tough stance and bold, direct talk, she'll need to mind her step now that she's dealing with leaders from around the world.