The Hunter's Backpack is an item kept in your inventory that can store 15 (or 45 if it's the Woven Hunter's Backpack) items attained from mobs after their death.
These include; String. Spider Eyes. Rotten Flesh. Bones. Arrows. Gunpowder. Ender Pearls. Gold Nuggets. Ghast Tears and Blaze Rods.
However, rare item drops, or non-vanilla item drops are not stored in the Hunter's Backpack. Like Zombie Brains, iron and gold ingots (from zombies and zombie pigmen respectively).