Funny this thread should appear - I was wondering the same thing myself today.
I didn't figure out what RA stands for but basically it allows you to change things such as traction control, ABS and best of all brake balance, during a race. You can set it up by going to Options and changing the controls. I use a DS3 so I just assigned some of the buttons I don't use to the RA functions. You do need five spare buttons for this otherwise you won't be able to use the functions properly in game.
The five buttons are: bring up the RA function menu, then select a function (with + & - meaning scroll up or down), and then change a function (with + & - increasing or decreasing the function). When you bring up the menu it appears as a transparent thing on your screen but as far as I'm aware you can't pause whilst you're changing settings, which is a bit annoying.
Anyway, I'm not great at explaining things but I hope this helps and perhaps someone who knows a bit more about it can shed some light. I'm glad I found out about it today, as I'd heard people talking about it before and it seems useful.
EDIT: Found this vid, not great but makes it clear enough: