The "automatic" refers to the winding mechanism of the watch - if it is being worn, then usual movement of the wrist will ensure the watch remains wound, hence the watch is being wound "automatically" as opposed to requiring manual winding.

The power reserve is around 48 hours at most, so if it hasn't been worn for a couple of days, then it will have run out of power and will require winding. I like to give them a few manual winds when they have been sitting around for a while, then wearing them takes care of the rest.