Quest items are items that have the main purpose of being used for completing a quest.
"Quest item" is also a specific property (or "flag" in programming terms) of an item in Fallout 3. An item with this flag cannot be dropped, sold or stored in containers. Contrary to ingame information these items do not actually weigh anything. Not all quest items in the general sense of the term have this flag.
The purpose of this setting is to prevent the player character from rendering quests incompletable by killing characters which are necessary for that quest or by selling or dropping items which are required for its completion.