These are spangle galls on an oak leaf.
There are several different kinds of spangle galls, very difficult to tell apart, but these are probably caused by the tiny common spangle gall wasp Neuroterus quercusbaccarum.
A tiny gall wasp (CC0) and common spangle galls by M J Richardson (CC BY-SA 2.0)
In June, the female wasp lays her eggs under the skin of the underside of an oak leaf, sometimes as many as a hundred on a single leaf. The chemistry of the egg seems to include instructions for the tree to make small, flat, round galls, one for each egg, in which the larvae live, eating the gall’s tissue but causing little visible damage to the leaf.
In the autumn, the galls detach from the leaf before leaf-fall, scattering pale confetti-like discs around the tree. The galls are then covered by the falling oak leaves and the larvae, safe under the leaf litter, continue to develop through the winter. They will emerge as adults in the spring.
Header image: common spangle gall by AnneTanne (CC BY-NC-SA 2.0) flickr.com