Rapid Shot works just like Quick Magic. It's a percentage chance for a Ranged Attack to occur with 0 Delay and 0 Reload ("Recast").

In regards to gearing for it, I personally do not. I gear for max DPS. Having this augment to Rapid Shot is just an added bonus, and to start having it kick in you only need at least 2 of the +2 pieces equiped. I still have like all of the Artifact set macroed in for the various job abilities.