When time elapses, it passes or goes by. If two hours elapse during a movie, that's how long the movie takes from start to finish. When a teacher says “five minutes have elapsed since the test began,” she means five minutes have passed.

The word elapse usually describes chunks of time in a measured, formal way. You probably wouldn't say “ten seconds elapsed while I tied my shoe,” but you might read in a mystery novel that “three days had elapsed since the detective last saw the suspect.” Scientists use it when tracking experiments: “After six weeks had elapsed, the plants had grown four inches.”

Elapse always refers to time moving forward. You can't say time elapsed backward, and you can't elapse anything yourself (you can't “elapse the hours”). Time simply elapses on its own, like sand falling through an hourglass. When a deadline approaches and you realize how much time has elapsed since you started a project, that's often the moment you realize you need to work faster.