Way back, when you visited a Disney park, there wasn't a "ride all day" admission. Everything was done ala carte.
Instead of purchasing an admission ticket, you purchased a book of ride tickets. Rides were broken up & the "E Ticket" was reserved for the best of the best attractions. Think Pirates, Haunted Mansion, Jungle Cruise, etc.
Now, the term is used to describe any new rides which are on the same "best of the best" level. Think Splash, Tower of Terror, etc.