In the case of sites and players an element (also known as an entity) within EnGage must first be Assigned to either a Site or Player. Once assigned, publishing make the assignment "live" overwriting the previous assignment. Publishing for sites and players can also be scheduled to occur at a later date/time.
In the case of content, publishing makes the content available for use within the EnGage (can be added to programs, playlists, or campaigns) and downloadable by the EnGage Player. If a content file is overwritten with a file of the same name, the newly uploaded file won't get downloaded by a player or updated in EnGage until it is published.