It's a site full of problems, which generally require some mixture of math and programming skills to solve. You can trade off between the two. If your math is good enough, some problems can be solved with pencil and paper. If your programming is decent, some can be solved by brute force search. There's no time limit, you don't show anyone your code, you just type a brief answer into a text field, so the only constraint on the efficiency of your code is how long you're willing to leave it running.
It's a lot of fun; the math involved can get pretty advanced on some of the problems.
