An alloy is a combination of materials to make a substance that has properties that are different from the original materials.
Aluminum by itself is too soft to stand up to the pressures a piston copes with. But if it is mixed with other materials, alloys are produced that are ideal as pistons.
In the same way, different steels can be made with a huge range of properties, by changing levels of carbon in iron.
Alloys can be designed with exactly the properties a particular part needs.