MEP is a multi experience pass. It is issued to you if you are holding a LL for an attraction that is down during your LL window. It is good for multiple attractions and is not time limited (it is day of only, though) the great flexibility of where and when to use it makes having one or two or five a great asset to touring.
To get one you can “fish” which means speculating on a ride that you don’t necessarily intend to use but hope to convert to MEp since that ride historically goes down often or issues more powerful MEP than others.
Also this means holding onto the LL for a significant portion of the window before modifying it. Do NOT let your LLs expire! But if fishing you will want to ride that line to maximize fishing.
However I prefer to speculate on already down rides:
Another way to fish is to see that a ride is already down and is issuing LL for a short time into the future. Booking this LL could convert to a MEP if the ride stays down, which might have good odds since it is already down.
Oh: another benefit of MEP is once it is issued* you can book a new LL. So when speculating on a down ride you can mine that same ride again and again as long as it stays down.
* assuming the issuing LL was your most recently acquired LL. The rules are complicated, but they are logical.