Valuable Garbage is a concept unique to video games -- movies, TV shows, and books rarely have scenes where characters regularly find fully stocked first aid kits or weaponry by rooting through a pile of trash. Within video games, however, it's remarkably common: major popular releases like Neverwinter Nights 2 and Borderlands have the player *constantly* locating priceless artifacts in dustbins, toilets, and piles of random detritus. Even celebrated franchises like The Legend of Zelda and Final Fantasy have a long running tradition of leaving cash money laying around in random clay pots, roadside crates, and clumps of grass.
By contrast, in a game like Fallout 3, virtually all useful found items are believable as lost or forgotten relics from a dead civilization. Bottle caps, packs of cigarettes, scrap metal -- all things which have value in the game, but could reasonably have been discarded as garbage before the bombs went off... although this still doesn't explain the occasional anomaly, such as an office desk stuffed to the brim with boxes of instant mashed potatoes. Valuable Garbage does not include miscellaneous crafting items which might be useful in the right hands, such as "coils of wire" or "brass belt buckle", but rather refers to objects of plain and unmistakable value within the game's setting.