This is an encoded string, used in API's where incremental sync of content is required. This encoded string contains the details of your current API call and when it is passed in the header, the server returns next new records based on resumeKey value. When resumeKey is returned as null it means there are no further records available to fetch. ResumeKey has an expiry duration of 14 days.
"056ec4c3-be6b-4c36-a4cc-a403ce9592a1"