The Backpack is an item that increases a character's carrying capacity. A Level 5 Fitness character can carry up to 120 lbs with the Large Backpack equipped. A Backpack is required to carry heavy building materials such as Concrete.
When increasing a Fitness level, either by levelling up or using Health and Fitness Guides, the carrying capacity will increase by 12 lbs. This is because increasing a Fitness level adds 2 lbs to a character's personal carrying capacity, as well as the 10 lbs per level provided by the Backpack.
— In-game Description