what really is pulse width modulation?

[askowronski]

PWM is going to be the key to taking the digital output from the arduino and turning it back into an analog signal.
the way i understand it now is that pulse width modulation is a frequency based square signal that gets output through a resistor to obtain a certain difference in potential.

how this goes from a square wave through a resistor at a changing frequency to give a -5 -> 5V signal is still to be understood.