A semaphore is a gadget used for conveying signals. Semaphores have either lights or mechanical arms that move.
The word semaphore is related to a Greek word meaning "signal," and that's exactly the purpose of semaphores: they give signals. The place you're most likely to have seen a semaphore is a railroad crossing: the gadget with moving arms is a semaphore. Some semaphores have mechanical arms, while others just use lights to communicate. A semaphore system can also use flags. In all cases, semaphores give visual signals that communicate something important, especially to people traveling.