When things sizzle, they make a sound like the hissing and popping you hear when you fry food in fat. Drop pieces of bacon in a hot pan and they'll sizzle.
A grilled cheese sandwich sizzles in melted butter, and veggie burgers sizzle on a hot grill. You can also use sizzle in a figurative way, when it's so hot that you feel like food in a hot pan: "I'm not going to the pool today, I'll sizzle in the heat." This verb dates from about 1600, and it probably comes from the imitative Middle English sissen.