It is an imperative verb, meaning it is a command to someone to sit down.
Here are some examples of how to use it:
* Садись, пожалуйста. (Sadis', pozhaluysta.) - "Sit down, please."
* Садитесь! (Sadites') - "Sit down!" (More formal)
* Садись, я тебе расскажу. (Sadis', ya tebye rasskazhu.) - "Sit down, I'll tell you."