Legacy content is the term given to content from an earlier game included among the resources of a later one using the same engine, despite not being used. In many cases this content is clearly not meant for the setting of the latter game, or is an obvious remnant from other, deleted resources. There are two games in the Fallout series that contain legacy content - Fallout 2 (content from Fallout) and Fallout: New Vegas (content from Fallout 3).
Because Fallout: New Vegas legacy content was simply carried over from Fallout 3, most of its core information remains unchanged, things like weight, value, alignment (for NPCs), and BaseID. Behavior may not remain the same, as certain quests or scripts may no longer be present, or the conditions which would make items or content behave in a certain way no longer achievable. Additionally, while many of these resources remain, often the textures associated with them are gone or changed.