Swelling is the enlargement or increase in size of a body part or area due to fluid accumulation [build-up of fluids like blood]. It can happen as a result of injury, infection, or an underlying medical condition. For example, when you sprain your ankle, it may start swelling as a response to the injury.
Very Simple Definition:
Swelling is when a body part gets bigger because of extra fluid. For example, if you hurt your arm, it might swell up.