Update Tuning.md

docs/Electrical/Tuning.md

@@ -4,4 +4,13 @@ This page talks about tuning the controller gains in PX-4. This is essential for
 
 We have been using the same guidelines as [PX-4's Tuning Guide](https://docs.px4.io/main/en/config_fw/pid_tuning_guide_fixedwing.html) and we recommend following the same process. Here we have added some more information and lessons learnt.
 
+## Background 
+
+In a UAS, a PID controller ensures that the UAS responds the way the pilot expects it to. A PID controller takes in the error signal in roll, pitch, or yaw (i.e. the difference between the pilot's input command and the actual behavior) and then performs some calculations so that the error is minimized.
+
+The PID controller has gains, or constant multipliers, corresponding to three calculations:
+*Proportional*: This gain is proportional to the error signal. Increasing the P gain will proportionally send a larger correcting signal if there is a large current error.
+*Integral*: This gain is proportional to the *integral* of the error signal. Essentially it corresponds to a large signal if the cumulative error is large.
+*Derivative*: This gain is proportional to the derivative of the error. It contributes to a signal that is proportional to the instantaneous rate of change of error which it finds as the difference between the current and previous error.
+