PID means Proportional, Integral, Derivative. It is the common name for a regulator using these 3 operations to stabilize a system. It is very commonly used for multirotors.
A regulator is a function applied to the error a system detects (the difference between the expected and the measured values) to stabilize the result when there is some noise (there is ALWAYS some noise).
- Proportional -> Error is multiplied by a gain G
- Integral -> The integral of the error is multiplied by a gain Ti
- Derivative -> The derivate of the error is divided by a gain Td
More information on the PID on wikipedia