your engine is made up from a bunch of pistons in a chamber which go up and down. all the pistons connect to a rod which they spin when they go up and down. gasoline is a liquid that can catch on fire and explode very easily. a spark plug is a device at the top of the chamber which creates sparks.

when a piston goes down, gasoline and air are sucked in to the chamber. then when it goes back up the spark plug makes a little spark that makes the gasoline explode. this explosion pushes the piston back down and makes the rod spin. the rod that they spin is connected to the wheels which makes the car move. these explosions happen thousands of times per minute and keep the engine spinning