Bubbles resulting from localised loss of adhesion, and lifting of paint from the underlying surface.
If blisters do not go all the way down to the substrate, remove the blisters by scraping and sanding, and repaint with a high quality waterbased paint.
If blisters go down to the substrate, remove the source of moisture if possible. Repair loose caulking, consider installing ventilation systems. Remove blisters as above, priming prior to applying the topcoat.
Source: The Paint Quality Institute
Aalto recommends WB Exterior Gloss, Satin or Low Sheen (for exterior substrates), or WB Interior Low Sheen, Waterbased Enamel Low Sheen or Watebased Enamel Flat (for interior substrates).