These are flat washers with a particularly large outside diameter in relation to the hole size. They are commonly used to spread the load across the fixing material. Also known as penny washers, mudguard washers or fender washers in the USA.
Zinc coatings prevent oxidation of the protected metal to create a longer lasting finish. It does this by forming a barrier and by acting as a sacrificial anode if this barrier is damaged. Zinc oxide is a fine white dust that (in contrast to iron oxide) does not trigger a breakdown of the substrates surface integrity as it is formed. The zinc oxide, if left intact, can act as a barrier to further oxidation, in a way similar to the protection afforded to aluminium and stainless steels by their oxide layers.