A booth is a small website to promote your project. You can create this on your own, or use the NCFC software to assist you. When registration is open, you can submit your booth (or create one with the NCFC site when those tools are available) and it'll be part of the convention as long as it meets the requirements. Forum threads are acceptable as booths as well, we've had a few of those before- but honestly I think there's a bit of fun in developing your own site for your game.

If you want some examples, just check out last year's booths.