A numbered card is a limited-edition card with the print run stamped right on it — front or back. You'll see something like "10/100," which means you're holding the 10th card out of a total run of 100. That number tells you exactly how rare the card is.
Numbered cards are short-printed by definition, and lower print runs mean higher demand. A card numbered to /25 is going to command a lot more attention than one numbered to /500. Collectors love numbered cards because there's zero ambiguity about scarcity — the math is right there on the card.