If this is the first you've heard of it, it's a contest where you get a Photoshop document and have two weeks to convert it into HTML and CSS. You're judged on an established set of criteria.
The design this year is by Daniel Mall.
You have to submit your design through CodePen. As you'll need to host files (like images) to make it happen, and you'll want to be working privately, you'll need a CodePen PRO account, which you can have for free for the duration of the contest. Just log in and then go here and get the free upgrade.