Backpacks have numerous pockets for storing items that you might need while adventuring and include industrial-strength straps for attaching additional items for easy access. Padded bands strap across the wearer’s chest and waist to evenly distribute the backpack’s weight, and it can carry roughly 2 bulk of items. When fitted properly and worn, the bulk of the backpack itself does not count against your bulk carried (though it does if you carry it in your hands), but the bulk of any items within it does.
When wearing a properly fitted industrial backpack, you treat your Strength score as 2 higher for the purpose of determining your carrying capacity. This does not stack with the effect of a consumer backpack.