When showing offensive statistics, one column heading is RF - where is the part of the website that tells me what RF means?

RF stands for Range Factor. See if you can find that somewhere in the help pages. You could also try searching for it in the forums to see if there' been some discussion about it.

You can also learn it about it here:

http://m.mlb.com/glossary/advanced-stats/range-factor

It is a stat that I have never looked at.

It's just Math that has little affect on the outcome of the game. Catchers and 1b will have a high range factor due to putouts and strikeouts. Right Fielders will have a low range factor because hitters less balls their way in HBD.

There are only two true uses for RF that I've found.

1/ The positional differences display how the HBD game is coded. The average amount of fielding chances handled by each position, relative to others. Obvious example, we know the RF handles the fewest chances, but when you realize even the greatest RF is going to handle less than two chances per game, that affects your strategy.

2/ If you have a player whose RF is significantly among the worst at his position, unless he's an excellent hitter, you might want to think about fixing that.

I think it's a pretty weak stat myself. It can be artificially high or low depending on whether a team's pitching staff strikes a lot of people out or not, groundball vs flyball ratios, handedness of pitchers, etc. Unless you can account for these things, it's rarely apples to apples when comparing two players even at the same position.