When you create a Mirroring Channel and select a "Start page", mirroring service will take all the updated/changed/new content underneath that selected start page (including start page if it's marked) and mirror it. If you want to mirror content from different nodes in the site, you can create multiple mirroring channels. See Creating a channel in user guide. 

Shahid

Is there a way that if all my pages are under root node. When I give the root node as the start page it gives me error saying root node cannot be a starting page. 

Thanks

No, you can not select Root node as your start page for mirroring. It's as-designed.