1. When you are in a group that is studying, working, and so on, and you are able to learn or work at the same rate as the others, you keep up or keep up with the group. Fall behind is the opposite of keep up.
2. When you keep up or keep up with people or things that are moving, you are able to move at the same rate. Fall behind is the opposite of keep up.
3. When you can understand and respond to a situation that is changing, you are able to keep up with the situation.