Appearance of a denser colour or increased gloss where wet and dry layers overlap during paint application.
Maintain a wet edge when painting by applying paint toward the unpainted area, and then back into the just painted surface. This technique (brushing or rolling from 'wet to dry' rather than vice versa) will produce a smooth and uniform finish. It is also wise to work in manageable size areas; plan for interruptions at natural breaks, such as windows, doors and corners. Using high quality waterbased paints makes it easier to avoid lapping problems because higher solids (pigments and binder) content makes lapped areas less noticeable. If the substrate is very porous, it may need a primer or sealer to prevent paint from drying too quickly and reducing wet edge time. Oil based paints generally have superior wet edge properties.
Source: The Paint Quality Institute