Pick a topic of your choice and make / preform / write about it in an almost university style. You have to also do some tedious stuff like a log of what you do each day, presentation and evaluation.
It's a good qualification and can give you something to talk about in your PS if your really stuck for stuff. However it's quite a bit of work, so you need to be motivated about your subject.
Don't worry about it until next year. Depending on how you juggle your as levels, it maybe a good / bad thing to take.