Electroplating involves depositing metal onto the parts, and that is not particularly difficult -- in fact we have a tutorial here to teach school children plating with kitchen ingredients.

... but getting the metal to deposit bright, hard, evenly, and free of porosity & pinholes is much more difficult  🙂

"Brighteners" (more properly called 'addition agents') are chemicals which are attracted to the components being plated much like the metal is, and which 'shield' the parts against the metal being deposited to some degree. They are helpful in evening out the plating thickness, but perhaps the principal thing they achieve is a smaller, tighter, harder, grain structure.

We don't want the plating to grow like rock candy, with large crystals. Rather, we want the brightener to very quickly shield a metal crystal while it is still very very tiny, so additional tiny crystals will form instead of existing ones growing larger. Countless millions of very tiny crystals is how you build brightness, hardness, and freedom from porosity. In some cases common chemicals like molasses, saccharin, and formaldehyde can be used as brighteners. But the best results are usually obtained as a result of suppliers spending years of R&D to perfect them, and then offering them as proprietaries. Some of the best brighteners can't be made by mixing available chemicals anyway, but only through synthesis from precursors (similar to how plastics or gasoline are made from crude oil).