Beanie Babies are simple, inexpensive plush toys filled with small beans instead of cotton stuffing. They became a fad in the mid-90s when manufacturer Ty began "retiring" certain designs. This created a collector's frenzy, since a few early Beanie Babies had become highly sought-after and valuable post-retirement. At one point, you could find racks of Beanie Babies in virtually every store -- whether the store normally sold plush toys or not -- as everyone tried to cash in on the craze. Collectors scoured shelves looking for that one rare Beanie.
Of course, a fad like that will always burn itself out. A collector's craze causes the manufacturer to make huge amounts of subsequent toys to satisfy demand. Collectors buy them up thinking they will increase in value. Since rarity is determined partly by supply, the huge production runs render later toys worthless. Disillusioned collectors leave the hobby, and the market collapses (a very similar situation happened to comic books in the 1990s as well). Today, you can still find the occasional sad pile of sun-faded Beanie Babies collecting dust in store windows.