It's the difference between how a team performs in a given situation and what their expected points would otherwise be. EPA stands for expected points added, so it's a measure of how your offense or defense performs above average.
The methodology is here https://pubsonline.informs.org/doi/pdf/10.1287/opre.19.2.541
As far as QB EPA (or offensive, defensive, etc you can adjust the filters) the 4th down bot has a good page you can play around with here https://rbsdm.com/stats/stats/