What is a strut charge?
In airliners, the landing gear shock strut (aka oleo, gear leg, etc) is filled with hydraulic fluid (H-515, MIL-H-5606, AeroShell Landing Gear Fluid, "red stuff"), then topped off with nitrogen.
The fluid provides the main shock absorbing function on landing, while the nitrogen smoothes out lesser bumps, eg during taxi.
To fill it, the strut is compressed (we use a platform that can be pumped up to three feet or so), then filled with hydraulic fluid. The strut is then "blown down" using about 50 psi of nitrogen. Once it's fully extended, the strut is charged with nitrogen - around 1200 psi or so.
The process is a little more involved than what I've described, but it gives you the idea, I hope.
The strut charge, then, is the pressure of nitrogen in the strut. It's supposed to correspond to the strut extension. If extension is too low, either you're low on fluid or low on nitrogen. If it's too high, you've got too much nitrogen (can't really have too much hydraulic fluid).
And for the other A320 AME's, yes, I know, upper and lower chambers, etc, but just trying to keep it simple.
The primary function of the design engineer is to make things difficult for the manufacturer and impossible for the AME.