An Account Item is a reward claimed from the second tab in the Email Window. Account Items can be claimed once by every character on an account. Stacks of Account Items are claimed all at once. Most of these rewards are from the Paragon Rewards Program and the Incarnate System.
There are some Account Items that can be only claimed by level 50 characters (Ascension Armor, for instance). To make these not show up in the list for non-50 characters, there is a setting in the Options Window to hide unclaimable rewards.