Fielding Independent Pitching
The core tenet of DIPS theory, as first proposed by Voros McCracken in the late 1990s, is that pitchers have minimal (if any) control over the outcomes of balls that are put in play against them. The two corollaries of this discovery, which has proven to be true in the most general sense (with a number of caveats and exceptions), are that a pitcher's Batting Average on Balls in Play (BABIP) will regress toward the league average - usually .300 - if given enough opportunities, and that the Three True Outcomes - walks, strikeouts, and home runs - are the primary indicators of actual pitching skill.
With that in mind, sabermetrician Tom M. Tango developed FIP, a DIPS stat that mirrors Earned Run Average but only uses BBs, SOs, and HRs. The formula for FIP is simply: (13*HR + 3*BB - 2*K)/IP + C, where C is a constant term that re-centers the league-average FIP to match its average ERA.
The weights for FIP were derived from the formula for batting linear weights, setting the DIPS events relative to the average value of a ball in play (essentially zeroing out the runs above/below average from BIP).
In FanGraphs' version of Wins Above Replacement, FIP is used as the basis of pitcher value, meaning FanGraphs assigns credit or blame to pitchers based primarily on their walks, strikeouts, and home runs allowed.