The subscribe action will check files that exist on both sides, and if changes have been made, the updated file will be downloaded. Updated files on the right are copied to the left if the file name already exists on the left.
For example, you like to keep a copy of a handful of important documents with you when you travel. New files are added only very rarely, but existing files are updated often, and you want SyncToy to make sure that the files in the left folder are updated with changes made to those same files in the right folder. Subscribe is the SyncToy action for you.
See also: