Disney Dollars were a form of official currency within Disney parks, issued by The Walt Disney Company starting in 1987. They could be spent 1:1 with U.S. dollars at Disneyland, Walt Disney World Resort, and Disney Stores.
While Disney stopped distributing them in 2016, they technically remain redeemable at the parks today. However, because they are no longer in production, they carry much more value as collector's items.
As a friend of a cast member, I had access to lots of extras.