A long-running operation (LRO) is an API method that takes a longer time to complete than is appropriate for an API response. Typically, you don't want to hold the calling thread open while the task runs as it offers a poor user experience. Instead, it's better to return some type of promise to the user and allow them to check back later.
The Google Drive API returns a LRO every time you call the download method on the
files resource to download the content of a file
either through the Drive API or its client
libraries.
The method returns an operations resource to
the client. You can use the operations resource to asynchronously retrieve the
status of the API method by polling the operation through the get method. LROs in Drive API adhere to
the Google Cloud LRO design
pattern.
For more information, see Long-running operations.