Imagine a railway intersection where only one train can pass at a time. If another train tries to pass the intersection while it's already being used, the trains will crash.

The same thing can happen in a computer. As soon as a resource gets changed by two different sources at the same time, things can go wrong.

In rail traffic, this is solved, by having a semaphore to only ever allow one train to pass the intersection. The same is done in computers. Here, a semaphore only ever allows one source to access a critical section. Everyone else has to wait, thus preventing crashes or any unintended behavior.