A fender-bender is a small car accident. Nobody is badly hurt. The cars just knock, often slowly, in a car park or in a queue of traffic, and you get a dent or a scratch. Friends say this; newspapers and police would more often say “minor collision”. It is informal, and it sounds especially American, though British speakers use it too. It can sound a bit light, even cute — which is fine when the damage is tiny. Though actually, it would be the wrong word, and a bit unkind, if someone was hurt. A fender is the part of the car over the wheel, the bit that often takes the bump. A bender, here, is “something that bends”.