Fish eye. Caused by any of several types of contamination on the surface of the wood that changes the surface tension of the finish over the area causing the finish to flow away from the contamination. Silicon is a primary candidate because once something is contaminated with it it's a bitch to remove.
Commercial fish eye eliminator will correct the problem but you have to be cautious with it since. It's usually a silicon and if you get it on anything else, including tools or applicators, that will, in turn, become contaminated and you enter a vicious circle.
Best bet is to seal it off from any subsequent layers is one or several thin and lightly applied coats of finish. Dewaxed shellac will work also. Saw an article on a couple of methods for doing so but can't pin it down right now.