Essential

Essential is an attribute which can be assigned to non-player characters in games which use the Gamebryo engine (e.g. Fallout 3). The item equivalent is simply called quest item which should not be confused with the general usage of that term.

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.

Characters
The main effect of this attribute is that the character cannot be killed, instead it will be temporarily rendered unconscious and wake up again later. In Fallout 3, all children in the game are essential characters, so even if you find a way to attack them, they still cannot be killed.

Items
"Quest items" cannot be dropped, sold or stored in containers. Contrary to ingame information these items do not actually weigh anything.