Above is good info- I may try to simple it down even more for a few concepts- and when time allows will find a few good videos that can help.

When you zero at 100 yards your scope and rifle are aiming at the exact same point. If you then dial your scope you are making an angular adjustment from the position of the scope. Think about a protractor... and how the lines get further away from each other the further you go. That is how mil/moa work- if you move your reticle 1" at 100 yards it moves it 2" at 200- and 10" at 1000.

So you are not really making adjustments in inches or cm.... but rather making angular adjustments (that do correspond to a given number of inches at a specific distance).

And how you use it is going to a ballistics calculator and finding your data. With a 6.5cm I have 315 inches of bullet drop to shoot 1000 yards (a slow factory load) so you need to dial 30 moa (8.7 mil) and then just hold on your center crosshair. Then you use the reticle in the scope to make adjustments. If you hit 30" low at 1000 yards it is hard to know what that looks like because how do you measure 30" through a scope at 1000 yards? So instead you just use the reticle (measuring tape) in the scope. You see that you hit 3 moa low (or however many tenths of a mil) and just make the adjustment with your scope.