From Eco - English Wiki

A Backpack provides extra storage space for a player. Every player has a backpack equipped when they first log in to a server. At a Tailoring Table a Player can craft Backpacks to increase their carrying weight, movement speed, or decrease calories consumed. The GUI for the backpack can be toggled by pressing B.

Any kind of item that can be stored in your toolbar can also be stored in your backpack (up to 16 item stacks).

All blocks cannot be stored in a backpack. They need to be carried by hand, or by using a vehicle such as a cart.