Lap marks occur when wet and dry layers of paint overlap during painting.
Lap marks can also occur when paint is applied in direct sunlight or in conditions that are too warm.
To avoid lap marks when painting you should paint in small enough sections to maintain a wet edge and apply paint toward the unpainted area and then back into the just painted surface.
Using a high quality acrylic latex paint can also help you avoid lap marks on the painted surface.
If you already have lap marks you can try diluting some paint and touching up the area again with the same paint or same color of paint you're using to paint the surface.