You don't need to worry about this unless and until the moderators tell you that you need to revise a hub you have written. When you publish a hub, if it needs to be revised it will automatically be unpublished but saved and an email will be sent to you stating why it needed to be revised. When you have revised the hub, you need to submit the revised hub for publication. I had a problem with this when I wanted to write a hub on Mom's artwork because it was elsewhere on the web. Because I wanted to focus on works where she'd won accolades or awards and her biography, specifically, revision wasn't practical and I simply deleted the hub. But had I wanted to revise it and talk about some other aspect of Mom's work, I could have. then this button would be important.