It is a "wishlist" of parents-to-be containing all the items they want for their baby on the way. A lot of stores like Target or Babies R us offer a baby registry. You will be given like a wand, scan the things you want for your baby as a gift. You will of course inform your friends where your baby's registry is. They will then go to that store and choose among your registry list there. Every time an item is bought it's removed from that wish list, I believe.
Baby registries are fun and very, very practical.