Final answer:
In the context of a REST API, the "read-only" key represents an array, which is a data structure holding multiple values indicating folders the user can access.
Step-by-step explanation:
In the context of a REST API, the "read-only" key is an array. An array is a data structure that allows you to store multiple values, which in this case are "Accounting Folder" and "Sales Folder". This indicates that the user has read-only access to these folders. An array is typically enclosed in square brackets, which you can see in the provided code snippet.