OP it's part of the new-ish Steam XP System. Collect sets of cards to "Level up" and besides getting a shiny number next to your Steam name, as well as a badge for each time you "craft" a complete set, it will also increase your chances to "earn" more Cards through random drops. There are multiple levels per game that you can craft. Beyond that, I don't really know what the XP System is supposed to do.
You can also sell the Cards on the Marketplace for some extra pennies (probably make a quid or three) if you CBA.
Since my initial batch of drops I have not had any further drops, and although I was tempted by the prospect of levelling up for a moment or two, I am non-plussed as I don't really see any benefits from spending some money on virtual items for virtual notions of advancement.
