A term I've encountered only with hiking, though I bet one could use maildrops for other things as well. Basically, when you're going to be on the trail for a while, you pack boxes of food, money, equipment, and any other stuff you'll need. Then you have someone back home send the boxes at a pre-ordained date. It reaches a small mountain town a couple days or so before you do, and when you get in town, you pick it up. It saves you from always buying food on short hikes, and is necessary on longer hikes to prevent carrying useless stuff. Usually you write on the box 'save for hiker'. Post office holds boxes for two weeks.