EDIT: OK Servers migrated but it still can't come close to handling HN rush levels. While that's passing I can explain the main concept here.
A task on codefights is a function written in javascript that has 1 bug on 1 line of it and your goal is to find and fix it as soon as you can. There is a Solo mode and a VS Friend mode. In the Solo mode you start with easy tasks and need to find the bug in under a minute, if you do, you go to the next rounds where the tasks progressively get harder.
In the VS Friend mode, you codefight someone else where you guys have 5 rounds facing the same task in each round and each correct solution is 100 points, pass is 0 points and an incorrect answer is -25. At the end of 5 rounds whoever gets more points wins.
Please let me know if anyone has any questions or feedback. Hope you guys will get to try it for real soon :).