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!
Blue '99 UZJ100 2" Lift, Replicated Outback stackable drawer system, Engle 63 QT, Custom Front Bumper, Gamiviti Shorty roofrack.