EPA stands for expected points added. It's a derivative of expected points, which in itself is a measure of how many points you can expect to gain relative to down/distance. For example if you have the ball on the opponents 20 and it's first down, you have an EP of say 4 (roughly)...if it was third down your expected points would be 2.5 or thereabouts. I believe expected points and EPA both draw from the same data set that win% and success rate do also.
EPA is just the difference between what your expected points is for a given play (or drive) and the outcome...like if it's fourth down and 10 from your own 40 (probably net zero or even negative EP in that position, so say its like -.15 expected points in that particular scenario) and you gain 11 yards that would be significantly positive outcome (like 1.25 EPA, I dont have the calculator in front of me) that play would be 1.10 EPA
there is an awful lot to be said on this but im not sure this is the time or place tbh