Watercooling is a technique for liquid cooling of the main components of a PC. A computer is commonly cooled by air through an aluminum radiator that captures the heat from the component (the processor, for example) and by a fan that dissipates the heat stored by the radiator. You can see for example our guide on PC fans.
What is watercooling?
Compared to aircooling, cooling components with water is more interesting in some areas. First, because of its physical properties as a heat transfer fluid, water conducts heat better. Then, water is denser than air: to cool an equivalent amount of heat, less water is needed. The heat generated by components such as the processor and graphics card is thus evacuated more quickly by watercooling.