Your character can collect a lot of items in Ring of Elysium, mainly ammo and weapon upgrades. They won't get removed from your inventory even after you switch to another weapon. This means that your backpack can be quickly overloaded with useless items.
To discard items, open the backpack and drag the items over to Nearby Loot column.
Close the backpack - items moved into that column will appear on the ground. Use this method to make room in your backpack or to share items with your friends.