BABIP: Batting Average on Balls In Play, also known as hit rate. Based on the theory that once a ball is hit the pitcher has no control over what happens, this is the measure of how many balls that fall inside the walls of a stadium fall for hits while a pitcher is on the mound. The league average BABIP is about .300, and while pitchers' marks can fluctuate from year-to-year, they usually end up right around the average over their careers.
LOB Percentage: Left On-Base Percentage, also known as strand rate. The proportion of baserunners allowed who don't end up scoring. League average is around 72 percent. Like BABIP, this can fluctuate as the result of luck.
HR/FB Percentage: Home Run/Fly Ball ratio. The proportion of fly balls a pitcher allows that end up as home runs. Some sabermetricians argue that this is also a product of luck; it can definitely vary based on things like the pitcher's home park and the potency of opposing lineups. League average HR/FB percentage is around 11 percent.
FIP: Fielding-Independent Pitching. An estimate of what a pitcher's ERA "should be" with a neutral defense behind him. Walks, strikeouts, and home runs allowed are the only things considered, so this number cannot be tainted by aberrant hit and strand rates.
xFIP: Expected Fielding-Independent Pitching. Same as FIP, but with an estimate of how many home runs a pitcher should give up (calculated with the pitcher's fly-ball rate and the league-average HR/FB percentage) instead of the actual number of homers allowed.