Run Expectancy
How many runs a base-out situation is worth on average.
How it actually works. There are 24 such states, from nobody on and nobody out through bases loaded with two out, and each has a known average. Every in-game decision can be checked against them.
Why it is done, and when it is wrong. This is the table that settled the bunt argument, the intentional walk argument and most of the stolen base argument. An out is expensive and a base is cheap, and almost every traditional small-ball play trades the cheap thing for the expensive one.
Batter's Eye: look up anything in baseball