RSVP is an abbreviation for répondez s'il vous plaît (please reply)
So basically when you reply to an event you do "reserve" your space in the game. It really just means to join the game. You can also enter 2 comments when your RSVP - the first one (public) will be seen on the Event page by all other users (so be careful what you write ;) - you can just say hello or something about the game or even "...".
The second comment goes to the Event Creator (for the PokerDIY Freerolls that is me) - so if you want to let me know something about that event privately then you can do this (but it's not mandatory).
I really need to put up some help text but I have not had time ;)