Mono-filament (mono for short) looks similar to a lace hair system, except that lace is made out of fibrous material, mono uses a plastic material to create netting. Therefore it is even more sturdy than lace, whilst at the same time being very breathable, due to the holes in the netting.
A mono base hair system is often referred to as a mono top hair system, because mono is usually used on the top area (i.e. the crown), and not at the sides because it doesn't create a natural edge, unlike a lace or skin front.