From Diablo Wiki
A Global Drop refers to an item drop or reward given to all players in the game, regardless of their location or participation in the activity or monster killing.
All drops and rewards in Diablo 3 are shared, or customized. Monsters drop different items for each player nearby, and players can not see or pick up the loot provided to other players. (A big change from Diablo 2's ninja loot gameplay.
To say a drop is global means that the monster will drop items or other objects for everyone in the game, regardless of whether they are nearby or not. This is most commonly done with KeyWarden farming in search of Infernal Machines. Even with a Global Drop, players still have to visit the location of the monster's corpse to click their items up off the ground, and players who are split farming keywardens in the same game will usually call out their success (or failure) in dropping a key (or Machine, after Patch 2.3) and wait for other players in the game to portal to them and pick up their key.
Many other rewards can be thought of as global drops, except they aren't actually drops, in terms of requiring players to visit the monster's corpse site to be rewarded. Things like experience bonuses for completing quests or Rifts, earning Achievements, receiving Horadric Caches from Tyrael, etc, go to everyone in the game, regardless of their location or participation in completing the quest or event.