Learn French: Reste calme avec ton chef. - Stay calm with your boss.

This sentence uses the imperative (command) form. For regular verbs ending in -er like rester (to stay), we drop the final -s for the tu (informal "you") form. So, tu restes (you stay) becomes just reste (stay!).

Does reste mean "rest"?

No, this is a "false friend"! The French verb rester means "to stay" or "to remain". If you want to say "to rest", you would use the verb se reposer.

How do we know to use ton instead of votre for "your"?

The verb reste is in the tu (informal "you") form, which means you are talking to someone you know well. Therefore, you must use the matching informal possessive adjective, which is ton (because chef is a masculine noun). If you were speaking formally to someone, the sentence would be Restez calme avec votre chef.

Does chef just mean the head cook in a kitchen?

In French, chef is a very common word for "boss" or "manager" in any workplace. While it can mean a head cook, it usually just means "boss" in everyday conversation.