Roller marks or stipple is the unintentional textured pattern left in the paint by the roller.
Use of incorrect roller cover
Use of lower grades of paint
Use of low quality roller
Poor application technique
Use the proper roller cover (avoid too long a nap for the paint and the substrate) and ensure adequate film thickness and uniformity. High quality paints also tend to roll on more evenly due to their higher solids content and levelling properties.
Another great tip is to use water to pre-dampen roller covers to be used with water-based paint. Be sure to shake out excess water before painting and don't let paint build up at roller ends.
Begin rolling at a corner near the ceiling and work down the wall in three-foot square sections. Spread the paint in a zigzag 'M' or 'W' pattern, beginning with an upward stroke to minimise paint spatter, then, without lifting the roller from the surface, fill in the zigzag pattern with even, parallel strokes.