Backpack (BetterStorage)

Backpack is a useful and easy way to increase your carrying capacity. A backpack can store up to 27 items. To open a backpack place the backpack on the ground and then right-click it. To equip a backpack in your chestpiece armor slot (you can't wear armor while wearing the backpack), break your backpack while sneaking. To unequip sneak and then right click the ground. Backpacks will drop their contents when they break or the player dies. In multiplayer you can use someones backpack while it is on their back as long as they are near you and facing away from you.

Backpack is one of several similar items we use to expand inventory space, this one is from BetterStorage mod. Forestry also has similar item called Backpack and Big Backpack.