This article contains speculation, information which has not been confirmed by a reliable source. The game is still in development, and all information is subject to change.
Backpacks are equip-able items that primarily determine the wearer's energy. The backpack also has slots in which you can equip different modules, some of which determine the player's skills, and others may affect things like run speed and jump height.
"... the backpack actually powers all of your abilities and even some of your weapons." 
As seen in the original gameplay video, packs can be dropped as loot by monsters in game. These packs may already have some modules equiped in them, and it is unknown if you can remove the modules from the backpack.