Themes, blocks, and content ... Oh my!
Yes, the Main page content block is where your content will render. Yes those visibility settings control on what page/content type/role will show that block content area. Typically, you want to leave this block alone since it's the main presentation of your content (what's the point of having a page if it doesn't have any content?).

It looks like the CorkedScrewer's default home page is more a demo template to demonstrate its block regions; you can't replace the images without overwriting the existing ones in its images folder.  It can vary based on the theme developer, but usually you can control a theme's slideshow images from its settings page in /admin/appearance.