The Cardboard Box is a block/item added by the “Cardboard Box” datapack. It allows the player to carry a block around in it’s inventory.
Right Click the block you want to pack up with the Cardboard box in hand. Just break the box do pick up the block.
Place it back down to unpack the block.
This item is crafted inside the Worktable which is a block added by Sallan’s Core.