tompoland PID stands for Proportional Integral Derivative
It’s a way of designing a (temperature - but it could be any other variable) process controller so that the output of the controller is a function of the instant error [i.e. proportional to Current Value - Target], of the error’s derivative [i.e. is the error expected to increase or decrease in the future based on its ‘instant trend’] and the error’s integral [i.e. has the error been increasing or decreasing over a certain time period in the past]. If properly calibrated, it’s significantly better at keeping a process ‘on track’ than a simple ‘on-off’ switch based on whether the Current Value is above or below the Target.
Note that there are at least 4 parameters that define a PID’s setup: the target for the variable under control, and constants for the Proportional, Integral and Derivative ‘parts’ of the error measurement. When @DavecUK says “you can’t adjust the PID settings” he is referring to the parameters other than the target (temperature in the most common case for a coffee machine). These can be important as they may need to be significantly different to provide stable and accurate control from system to system.
If you want the theory, history and maths, this Wikipedia article has probably more than you want to know; if you have any specific questions, do ask!
