Imagine you're the Doorkeeper of a very important party inside your chest. The name of this party is "Your Life."
Would you let a dirty, mean monster who breaks everything into that party? Of course not! You'd stop it at the door. Today's proverb says our heart is that party. Bad thoughts (like "I'm dumb," "nobody likes me," or anger) are the monsters. If you let them in, they ruin the party. "Guarding your heart" means being an alert doorkeeper and saying: "Hey, bad thought, not here! Shoo!"