In this context, "mind" means "pay attention to". Every time we tackle a new concept in class, there is a gap between our current base of knowledge and the knowledge we'll need in order to show mastery of the topic. For examaple, when we began studying adjectives, you might not have known much about them. On the other hand, you might have had some background knowledge. In either case, there was some information on adjectives that you needed to learn - a gap.
How do we mind the gap? What specific things can we do to gain knowledge? If you knew that there would be a test on prepositions in only a month, what might you do in order to close the gap between what you currently know and what you might need to know in order to pass?
Extra fun: Where are "Mind the Gap" notices most commonly seen?
