Polyester is a chemical fibre produced from synthetic polymers, during a melt spinning procedure. It is a very durable and elastic material, which has the additional benefit that it is resistance to mould. There are several different polyester types, depending on the field the material will be used in. The variability is the reason why polyester has established itself as the most popular chemical fibre. It is also the most produced and sold fibre in the world.
You can find more information about Polyster (PL) in Wikipedia.