RF Gain is like your squelch on VHF. As you adjust it down, it squelches out (or quiets out) all but the strongest signals that can get past the S-meter setting which is directly related to your RF Gain setting. On my Yaesu FT-857D, as I adust the RF Gain up, the S-meter needle goes up from 0 incrementally. If I set the RF Gain to where the S-meter is at S6, then only signals that my radio receives that are greater than S6 will be sent to the audio. Otherwise, they are muted. IMVHO, it is a great way to eliminate some of the "noise" associated with operating mobile.
I always get Ø nose level with my vehicle's motor off. Once it is going, my usual S meter noise goes up to around S3. As I pass certain areas in my neighborhood or town, I get varying levels of noise, driving by lots of high power lines, places with a lot of Wi-Fi, etc. So I almost always keep my RF Gain at a level where only signals greater than S3 in strength will be heard. I had done a lot of work to my 80 to abate noise, and only on really bad propagation days do I need to open up the RF Gain more to even hear stations.
Keep the questions coming!