Changes how fast the gun can shoot. It's personal preference, 17 isn't too fast. I would keep it all the way up unless your field has rules that differ. I would never have a gun ramping above about 18.
Sorry, but you mean to tell me you have put around $200 into your Ion and you don't know what ROF means?
A good rule of thumb is to set your ROF to about 1 or 2 bps below what your hopper is capable of feeding, so that if anything should happen to the eyes, chops still won't be a major problem. For instance if you have an egg with y-board, set the ROF to about 15-16 bps. If you have a halo, go ahead and CRANK HER UP! Especially if you have a revy, don't set it higher than about 13-14.