The autopost feature gives users the ability to aggregate blog posts using any RSS feed. In other words, if an RSS feed is particularly relevant to any group, users can choose to automatically post that feed directly to the group. Instead of an RSS reader updating users, autopost will post any new updates to the group.
This is useful for organizations who want to have a single group which aggregates posts from all of the other groups on the site, or if a team using one group wants to be updated when another team in another group posts. This feature works with custom RSS feeds, so you can narrow what content you want.