I tried doing a search but couldnt find what i was looking for.
First question, what is the IMU that i see everywhere on the forum? What does the calibration do exactly, i know it calibrates it but what is it actually doing for the AC, and how often shoukd i be doing it on my P4P+ ? Along with IMU calibration how often should i be doing compass calibration. I have been doing it every flight. When i do it i follow the steps, horizontal and nose down. After i do the nose down it goes out of the calibration screen, does that mean its completed,  it doesnt give me any feedback like calibration successful. Im just curious if its suppose to say something when its done or if ita good it just exits and im ready to go. Thanks for the input.

when calibrating the compass you watch for the lights to change, when it is done it will exit calibration.  IMU should not need to be done every flight, it calibrates sensors and the gimbals but I think you should cal the IMU anytime there is an update on app or drone.