Take a read of rafcbat.wordpress.com
Not my website and it's a little bit outdated in terms of the procedure on the day, but is very informative regardless and will give you a nice overview of how the process works, and the tests you will be doing. Also make sure you have signed onto your careers portal and downloaded your info PDF as this is the up to date official guide from the RAF.
As an aspiring RAF recruit I have no doubt you'll love the day - just take it for what it is, a night on an RAF base where you can get to know some new people and have the experience of taking the CBAT, which is something that not everyone gets the opportunity to do.
Best of luck!