Polypoid basically means "having or resembling polyps".

A polyp is a spheroidal, solid mass that protrudes upwards or outwards from a normal surface.

A cyst, on the other hand, is an abnormal sac that is filled with gas, fluid or semi-solid material. This material can cause the sac to protrude upward or outward - and thus cause the cysts to resemble polyps.

The cysts occur on the serosa of the uterus in aged dogs that have had multiple pregnancies. When the uterus involutes, little bits of mesothelium are pinched off and fill with fluid and form cysts.