A CPU fan is a fan that sits on top of a computer processor. It helps keep the processor cool by pulling hot air away from it. CPU fans are also known as CPU coolers or heatsinks. 
CPU fans work by: 
Pulling hot air away from the processor
Dissipating the heat produced by the processor
Moving the air through the heat sink to cool specific components
CPU fans are important because they prevent the heat generated by the computer from disabling the delicate electronics. Maintaining your computer's cooling systems can increase their lives and reduce the chance of damage to important business systems. 
There are both air and liquid CPU coolers. Both types operate on a similar principle, absorbing heat from the CPU and redistributing it away from the hardware.