The Season, Game and Streak Finders each include the ability to search by Win Probability Added (WPA), Run Expectancy (RE24) and Leverage Index (LI) stats. The framework for calculating these stats was provided by Tom Tango, author of Inside the Book and the currently the Senior Database Architect of Stats for MLB Advanced Media. More information on his methods can be found in this article.
Win Probability/Expectancy is the percent chance of a team winning the game based on the current score, inning, outs, runners on base, and the run environment. These percentages are calculated using historical data.
Run Expectancy is a similar concept, but instead measures the number of runs a team is expected to scored based only on the number of outs and the position of the runners on base.
Leverage Index is a related concept that measures "how important" the plate appearance is, in relation to determining the outcome of the game.
Additionally, our Pivotal Play Finder, which allows you to search the most "game-changing" plays in MLB history, includes Championship Win Probability Added (cWPA) and Championship Leverage Index (cLI). These stats are similar to the ones above, except that instead of measuring the importance of a particular play to the outcome of that game, they measure the importance of winning that game to a team's chances of winning the World Series.
For more detailed information on Win Probability and its related stats, plus a complete glossary, check out this page on Baseball-Reference.com.