The analogy of the briefcase. You have two briefcases and they both have the same items inside. Those items are exactly the same but the briefcases are their own object (being in object mode). So you can move each briefcase individually, rotate each briefcase, scale them, and their items inside will stay the same. But once you open a briefcase (entering edit mode) and change their stuff inside, the changes will propagate to both briefcases items, since their are the same items they have inside.
