The Average Progress indicates the average percentage of how far your viewers got when viewing your file.  For video and audio files it is a simple average of how far people get.  For documents, it will show the average percentage of pages read by users.  For example, if a user reads the first page and the last page of a 100-page document that means they only read 2% of the document and then we average that percentage across all users that have viewed the file.