When a laptop is used for extended periods, it should sit on a solid surface so that its feet keep the bottom of the laptop clear. Most laptops draw in cooling air through slots on the bottom, and if the laptop sits on a soft surface e.g. a pillow or your lap, these slots would be blocked, resulting in overheating.
Even those laptops that do not have cooling vents on the bottom, partially rely on some air flow around the case to help dissipate heat, and could become uncomfortably warm if kept on your lap.