I'm just a novice baseball card collector, but I'll take a stab at it. Most sets of baseball cards have "parallel sets" that are serial numbered. For example, in recent years, the regular Topps set has had a "gold parallel" set that is numbered to the year that the card was produced. I have a Jonathan Lucroy rookie card from this year's Topps set that is #'d 1563/2010. The gold parallel cards are randomly inserted in packs of regular Topps at a ratio of 1:6 (1 card in every 6 packs). These #'d parallel cards can be fun to collect for two big reasons. First of all, a parallel set can be more challenging to assemble than a regular base set. Secondly, for people who collect cards of their favorite players, a #'d card is somewhat rare and is a neat addition to a player collection. Higher end sets of baseball cards have several parallel sets- some of which are extremely low runs. For example, in the set in question, "Bowman Draft Picks and Prospects," there are sets serial #'d to 299, 199, 50, 25, 5, and 1. In this set, a card that is serial #'d 1/1 is a one of a kind card known as a "superfractor." The Jonathan Lucroy superfractor, which is also considered a rookie card, is currently going for $23 on eBay. I wouldn't be surprised if it hit $50 before the auction closes.