In fantasy baseball, for pitchers I like using ERA, FIP, xFIP, BABIP, and xwOBA-wOBA. I used to use Siera...but have stopped with the advent of statcast. I also like trying to find pitchers with high strikeout percentages and low walk percentages. For batters, I like using BABIP, wOBA, xwOBA-wOBA, and k and bb percentages. FIP - basically looks at predicting ERA by with items pitchers can control. K, BB, HRs. xFIP - same thing but uses flyball rates rather than straight home runs. BABIP - tries to calculate luck. 300 is generally average. wOBA - a more advanced OPS weighted correctly to runs from an event happening. .330 is average (essentially replacement level). xwOBA - expected wOBA calculated off Statcast data. Subtracting these 2 can show if a player is doing what is expected of over/under performing. I use Fangraphs for most this stuff. It summarizes everything right there for you. I recommend going in more depth than my simplified summary with the glossary as others have said. Anything with an x in it implies statcast and you need to use a site like Baseball Savant to get data from it. You need to do statcast searches to get the good data. Directly typing the player name on the website does not show xwOBA. I'd be happy to explain more if you have specific questions. I'm sure there are people that understand it more than I on this site.