The word gag has several meanings:

1. A piece of cloth or other material tied over someone's mouth to prevent them from speaking or calling for help. In adventure stories, villains sometimes gag their captives to keep them quiet. To gag someone means to put a gag on them.
2. To choke or retch, that uncomfortable feeling when something makes you feel like you might throw up. You might gag on food that went down the wrong way, or gag from a terrible smell. Some people have a sensitive gag reflex that makes them gag easily when something touches the back of their throat, like when the dentist uses a tongue depressor.
3. A joke or comedic bit, especially in comedy shows or movies. Comedians work on their gags to make audiences laugh. A running gag is a joke that keeps coming back throughout a show or movie, getting funnier each time. In old silent films, physical comedy bits were called sight gags because they relied on what you saw rather than what you heard.