The tool, your thumb, is used to slide the nice and neatly lined up ammo, in to a magazine. Keep in mind, someone will tell you its ok to shoot 5.56 in a 223 (when actually it is ok to shoot 223 in a 5.56, but not other way around), and if you have a bolt action rifle with removable magazine, those magazines most likely won't load this way, but rather require the bullet to be slide back in the magazine, one at a time.
Mags for the AR, you should be able to line up that clip, and slide the ammo in the magazine. May take some practice.