Almost all vehicles (including hybrids) use internal combustion engines that burn fuel. In an internal combustion engine, air and fuel are ignited inside a cylinder and the pressure that is created forces pistons to move up and down. This rotates the crankshaft, which delivers power to the wheels via the drivetrain.
A piston is a cylindrical piece of metal that moves up and down inside each cylinder, compressing the air-fuel mixture and capturing the force created by the combustion process. This force is then transferred to the crankshaft, via the connecting rod.