Take a look at this page:
http://ebldc.com/?p=48Specifically the RC PWM section.
On the GCS you find Min, Neutral and Max values for the Inputs (Values sent to the Flight Controller from the Radio) and for the Outputs (Values sent from the Flight Controller to the ESCs or Servos).
On the Inputs the Min and Max values represent the Endpoints of each channel of your radio and the Neutral is the value your radio channel sends to the FC when you let go of the sticks.
As you can see the Neutral values for Yaw, Roll and Pitch are close to 1500 and for throttle is close to 1000.
On the Outputs the Min and Max should be 1000 and 2000 for ESCs and the Neutral should be the minimum value at wich the motors spin. For Servos they represent Endpoints and Center Value*
A piece of advise: You don´t have to fully understand this topic when starting, but if you have more questions don´t hesitate to ask over here.
* Not entirely true for every case but in sake of simplicity bear with me.