The PI-Controller is a combination of a proportional and integral controller.
The proportional term produces an output value that is proportional to the current error value. The contribution from the integral term is proportional to both the magnitude of the error and the duration of the error.
The idea is that you first set Ki value to zero and increase Kp until the signal oscillates then set the value of Kp to roughly half. Then increase Ki until any offset is corrected quickly enough for the process.