A dent is a gouge or hollow that's left in a surface after some kind of blow. If you hit a soup pot with a hammer, it will leave a dent.
Car accidents, whether they're serious or just fender benders, often result in dents. If you fall off your bike, you may get a dent in your helmet, and a mishandled package might be delivered to your door full of dents. There's also a figurative kind of dent, a reduction in amount: "You barely made a dent in that oatmeal — aren't you hungry?" Dent comes from Middle English, originally a variation on dint, "a blow dealt in fighting."