The CPU and other electronic components inside any computer produce heat as they perform the work they were designed to do. In a desktop computer case, the heat sinks mounted on the CPU and other chips, and the cooling fans inside the case, move the overheated air away from the chips before it can damage the components. Obviously, there's much less space inside a laptop computer, but the same problem remains: the heat has to go someplace.
Most laptops use two methods to dissipate the heat produced by the CPU: they have one or more air vents in the case that allow the heat to escape (helped by small fans), and they have a metal plate inside the bottom of the case that conducts heat from the motherboard to the case itself. You know how effective that heat-spreading plate is if you have ever actually used your laptop on your lap.
Because space is so tight inside the laptop case, it's not possible to add extra cooling fans to reduce the heat; the best you can do is to make sure there's nothing blocking the air flow. The air intake vents are usually located on one or both sides of the computer case, with the exhaust vents on the bottom, so cool outside air comes in through the sides and warm air exits through the bottom. You should take these preventive steps to keep your laptop from overheating:
Some laptops have a filter inside the intake vents to keep dust and other crud out of the case, so it's essential to make sure the dust doesn't accumulate in front of the filter. Use a small artist's brush or a can of compressed air to remove dust from the vents.
Be sure there's some free space underneath the bottom of the computer so the heated exhaust air has someplace to go. If the computer has flip-up feet, use them. If it has permanent feet attached to the bottom, they should be adequate if the computer is on a solid surface like a tabletop.
Don't use the laptop on a soft surface such as a bed or a couch: the weight of the computer could be enough to block the exhaust vents.
Don't use the computer when it's still in its case unless the case allows some free space for airflow to and from the vents.