A piston is a key component in a reciprocating engine. It's made from a cylindrical piece of metal with piston rings that help to form an air-tight seal once the piston is installed within the engine cylinder. The piston is attached via a piston pin or gudgeon to a connecting rod, which in turn is connected to the crankshaft.
In four-stroke (petrol and diesel) car engines, the intake, compression, combustion and exhaust process takes place above the piston in the cylinder head, which forces the piston to move up and down (or in and out in a horizontally opposed - or flat - engine) within the cylinder, which causes the crankshaft to turn.