It's a writing challenge basically. It's a virtual camp, and it's free. You have thirty days to do a project start to finish. You set your own goals, pick your genre, etc.
They have "cabins" where you can connect with others and share about your goals and progress with.
It's a spin-off from their main event which is NaNoWriMo. It takes place in November. NaNoWriMo is an abreviation of National Novel Writing Month.