A "gap" is a hole or blank area between two things. There can be gaps between:
- the rooftops of two neighboring buildings
- the doorway of a train and the station platform
- a person's teeth
- rich people and poor people
- what someone remembers and what really happened
When you ride a train, you'll often hear announcements that say "watch the gap". This means that you have to be careful when you're getting off of the train, because there's some empty space bewtween the station platform and the door of the train. It's very dangerous if you fall in this gap, so you have to be careful of it.
You may also hear the phrase "mind the gap", which means the same thing.