The heatsink when used in the context of electronic devices is a metal structure, usually composed of aluminum fins and copper heat pipes, that wicks heat away from parts such as the CPU, GPU, and other hardware components that generate a lot of heat.
Heatsinks help cool electronic components in two ways: active and passive. Active cooling attaches a fan to the heatsink to dissipate heat more efficiently at the cost of noise, while passive cooling simply uses the heatsink by itself to dissipate heat at a lower efficiency.
Enthusiasts with high end, overclocked components will sometimes opt to use water cooling. Whereas air cooling uses a fan attached directly to the heatsink itself, water cooling uses a circulating hydraulic system to carry heat from the component to a device similar to a heatsink called a radiator which is then usually actively cooled with a fan.
Hysteresis in mechanical keyswitches occurs when the reset point on the switch is higher than the actuation point. For example, the Cherry MX Blue switch actuates at around 2.5mm from the top, but its reset point is only 1.5mm from the top. This means that the user would have to release the key beyond the actuation point before another keypress can be registered. Hysteresis is one of the main reason why many gamers prefer linear switches since the reset point is almost exactly where the actuation point is.