Unintentional textured pattern left in the paint by the roller.
Use the correct roller cover - avoid too long a nap for the paint and the substrate. Use quality rollers to ensure adequate film thickness and uniformity. High quality paints tend to roll on more evenly due to their higher solids content and levelling properties. Pre-dampen roller covers used with waterbased paint. Don't let paint build up at roller ends. Begin rolling at a corner near the ceiling and work down the wall in metre-square sections. Spread the paint in a zigzag 'M' or 'W' pattern, beginning with an upward stroke to minimise spatter, then without lifting the roller from the surface, fill in the zigzag pattern with even, parallel strokes.
Source: The Paint Quality Institute