A PID is the digital control seen on some premium coffee machines which display the temperature of the brew water. It stands for Proportional-Intergral-Derivative. On a basic level, a PID controller uses the PID algorithm to determine the best way to control whatever process it's used for. A traditional thermostat has a tendency to hit a desired temperature, then turn off the heating element as the temp rises above its target. Then it'll kick back on as the temperature falls below the target. This results in uneven temperatures that can result in inconsistent shot quality in an espresso machine. There are ways to mitigate this with many machines, but it often means learning how to ride the temperature wave with your specific machine. This may require timing the heat-up time precisely or running water through the group head before pulling a shot. PID controllers use the PID algorithm to keep your machine at the proper brew temperature. This also means the you can directly control the temperature of the machine. While not true in every case, PID controllers are usually visible on the machine. They also usually feature control buttons to increase or decrease the brewing temperature. While this won't matter for most, for some home baristas, experimenting with different roasts and temps is key, and very rewarding!