Augmentation

Hello, new to PSO2, but not the world of PSO2. Can someone explain to me like a 5yr old on how auggies work? I traded ex cubes for an auto rifle, lvl 51 ranger. It has three augmentations with mastery of III each. I've augmented an older weapon but lost stamina III due to not knowing W.T.H. I was doing. Any info or help would be great.