What exactly is it? I understand there is something about code puzzles but how does it work? How does the leaderboard work - presumably there is some sort of scoring involved?
Daily two part puzzles, for one star each, 50 stars total. Second part is usually related to the first (but unknown until you solve the first) which makes you refactor or learn to write your code generically the first time. Input is usually a small text file. Puzzles are often dependent on earlier ones (spoilers: most years you write a interpreter for a made up language over a few days and later puzzles use it as an input generator which you seed with the text file)
Leaderboard points are only assigned if you're in the top 100 for either parts which you should ignore if this is your first time since it's not the point!
You get points for being the fastest to submit the correct solution to a puzzle part each day. First one gets 100 points, second one 99, and so on down to place 100. So you could get up to 200 points per day if you solve both parts of the puzzle as first one. However unless you can be on your computer when the puzzles unlock (midnight EST (UTC-5)) I wouldn't bother with the global leaderboard, it gets filled within minutes.
Leaderboard points are only assigned if you're in the top 100 for either parts which you should ignore if this is your first time since it's not the point!