A person who protects something is called a guard. You might see a security guard at the mall, a crossing guard on the street outside a school, or a palace guard at Buckingham Palace.
To guard something is to watch over it or keep it from danger. You should ask someone to guard your backpack when you go to the restroom at the restaurant — or take it with you. The verb guard also means to protect against unintended consequences. To guard against overeating at the party, you could have a light snack before you go, so you aren't as tempted by all the delicious snacks and desserts.