Flaking is paint that lifts and chips away from the surface in small pieces, usually because adhesion failed somewhere below. Loose paint has to be scraped off completely and the surface prepped before repainting, or the new coat will flake too.
Flaking often starts at one weak point, a missed spot in the prep or a coat applied over a surface that wasn’t dry. Once it starts, moisture works under neighboring paint and spreads the damage, so early repair keeps the problem small.