Formation of bubbles (foaming) and resulting small, round concave depressions (cratering) when bubbles break in a paint film, during application and drying.
All paints will foam to some degree during application, however higher quality paints are formulated so the bubbles break while the paint is still wet, allowing for good flow and appearance. Avoid excessive rolling or brushing of the paint or using paint that is more than a year old. Apply gloss and semi gloss paints with a short nap roller, and apply an appropriate sealer or primer before using such paint over a porous surface. Problem areas should be sanded before repainting.
Source: The Paint Quality Institute