You are confused. If it is first and 10 at the 50 yard line, there is still historical data on the expected points teams for teams based on that down, distance and FG. If historically they ended up scoring a TD 20% of the time, a FG 30% of the time or nothing 50% of the time, the EP would be 2.1.
If a QB then throws a 20 yard pass and it is now 1st and 10 at the 30, you can then calculate the expected points for a team historically at the 30 yard line and then the difference between the EP at the 50 and the EP at the 30 would be how many expected points the QB added. So for every down, distance and FP, you calculate the EPA by a given player based on the new EP of the new down, distance, FP that a give play yielded.
Also, PFF doesn't use this. Separate system. I think what Glider was saying is that PFF was claiming their grading was consistent with this EPA model not that they use this model. In any event, you asked what EP was and I have now explained it to you.