To fall behind means to fail to keep up with others or with where you should be. When you fall behind in class, you're not understanding the material as quickly as your classmates, or you haven't finished assignments that were due. When a runner falls behind in a race, other competitors pull ahead while they lag back.

The phrase suggests a gradual slipping: you don't suddenly appear at the back of the pack, but rather lose ground bit by bit. A student might fall behind in math after missing a week of school, finding that everyone else has moved on to new concepts. A construction project falls behind schedule when workers can't complete tasks as quickly as planned.

You can fall behind in all sorts of ways: in your homework, in paying bills, in a game of tag, or even in reading a book series that all your friends have finished. The key is that there's some standard or expectation you're not meeting, whether set by others or by yourself.

The opposite would be to catch up, which means working to close that gap. If you've fallen behind in your reading, you might spend extra time with your book each night until you're back on track. Falling behind isn't permanent: it's often a problem that can be fixed with focused effort.