ERA Minus, FIP Minus, and xFIP Minus are the pitching version of OPS+ and wRC+: a simple way to tell how well a player performed in relation to league average. All of these statistics have a similar scale, where 100 is league average and each point above or below 100 represents a percent above or below league average. However, as lower is better for (almost) all pitching stats, a lower ERA- or FIP- is better.
For example, Josh Johnson led the majors in 2010 with a 60 FIP-, meaning his FIP was 40% better than league average. On the flip side of the spectrum, Dave Bush had a 130 FIP- that season, meaning his FIP was 30% worse than league average. If the acronyms seem funky, realize that you don’t have to use them in order to get a point across. “Such-and-such has had an xFIP 20% better than league average” or “So-and-so has performed 10% worse than league average, according to fielding independent metrics.”
These statistics are all park and league adjusted, so they account for the fact that some pitchers throw in Fenway Park while others throw in PETCO Park. These adjustments also make it possible to compare pitchers between years and time periods.
For information on how these stats are calculated, see this article at “Walks Like A Sabermatrician”. In that article, ERA- is described as aERA.
Links for Further Reading: