The rucksack allows for extra item storage (27 slots), where you can store any item (except for the rucksack).
It is based on the Backpack (giving 9 extra slots), which inturn is based on the Pouch (that gives 3 extra slots).
The rucksack can be used in two ways.
Firstly the storage in the rucksack can be accessed by right-clicking while holding the rucksack.
Secondly the rucksack can be used in the Camping Inventory. When a rucksack is placed in the backpack slot of the Camping Inventory, the rucksack's inventory will be accessible as extra inventory space thus expanding your basic inventory by 27 slots! This allows for very easy and quick item management.