I keep hearing about superheat this and superheat that.

What is it, how is it used, and how do I calculate it? (In SIMPLE easy to understand terms please)

What do I do with the resulting information? How much is too high? How much is too low?

All my past work was done with a guage and thermometer - if the pressure seems ok (I use the scale on the guage or chart versus the pressure indicated for typical refrigerants) and if it's cold enough for the aplication, then I leave it alone.