Autopromote creates a new permission, "write automatically-promoted content". Administrators can select the content types for which this permission will be activated. Once those content types are specified, if a user who has the "write automatically-promoted content" permission creates or edits a node belonging to one of the auto-promoted content types, then that node will automatically be promoted to /node.
The classic usage for this module is a community blog site. For such sites, an administrator would create a "front pager" role, and gives that role the "write automatically-promoted content" permission. The administrator designates the Blog content type as an automatically promoted content type. Thereafter, all blog posts by front pagers automatically appear at /node.