The piston is a moving part of the combustion chamber. It is responsible for converting the energy released during the combustion process into mechanical work. The piston also performs a number of other important tasks:

It seals the combustion chamber
It guides the connecting rod (in trunk piston engines)
It dissipates the heat generated in the combustion chamber
It supports gas exchange (by means of gas suction and emission)
It supports mixture preparation with a special piston surface design on the combustion chamber
side which is known as the piston crown.
It houses the sealing elements (piston rings).