Summary
Items can be used by Survivors and are equipped on the Loadout screen - unless specified otherwise. Items generally help the Survivors in-game to ease their way towards escape. Depending on the Item, it is used in different ways. Items can be acquired either in-game or in the Bloodweb.
Items are kept as long as you manage to survive a level, if you die, you lose your item. Items are also recharged if you manage to survive a match and escape with them.
Add-ons are modifications that can be tied to an item, improving their use, making it even more powerful. Add-ons can be gained in the Bloodweb.