Switched Mode Power Supply : Voltage Mode Control and Error Amplifier Output
It's hard to explain a control system without control theory, but Vc will typically be the output of the compensator, which is the error signal modified by the compensation.  

This goes into a modulator.  So this voltage basically "sets" the duty cycle of the PWM.  So working backwards, whatever the output voltage is requires a certain duty cycle.  In the case of a buck converter (in CCM) it's Vout/Vin.  Drawing current from the output would normally cause the output to drop due to series impedances, but the control loop will adjust the duty cycle by moving Vc up slightly to keep the output at a constant value.

The compensation adjusts the response of the controller so that Vc gets to just the right point in a reasonable amount of time without overshooting the mark or oscillating.