I was told it was a credit tied to my room, and my understanding is it can be used on anything you can charge back to your room. Below are clips from the email I received with the offer.
A Special Offer for Eligible Guests
Starting Monday, March 30, 2020, eligible Guests of the Disneyland Resort Hotels may receive a $50 per night resort credit* for stays between June 1, 2020 and December 31, 2020. In addition to the resort credit, eligible Guests may also take advantage of a special Disneyland Resort offer to save up to 35%** on stays most nights June 19, 2020 through October 3, 2020.
The resort offer is valid for Guests who booked a room reservation, or a Walt Disney Travel Company vacation package, to stay at a Disneyland Resort hotel during the period from March 16, 2020 through May 31, 2020 and on dates that the Disneyland Resort theme parks are closed. Offer is not valid for room reservations or packages cancelled before March 1, 2020.
*Resort credit is valid for select food and beverage, spa services, and merchandise from any Disney owned and operated location at the Disneyland Resort that offers room charging privileges to Disneyland Resort hotel room accounts. Resort credit is not valid for Disney Gift Cards and other select items; please ask about eligible items at participating locations. Room credit will be applied against charges made to the room account, and Guest is responsible for any balance due after the credit has been applied. Not valid for payment of room rates or associated fees or taxes, must be used during the Guest's stay, and expires on day of check out.