That would bring you to this page:
Subdomain: This is the subdomain name, in the above example, it is 'thisisasub'.
Domain: Your domain, you can leave this as is.
Document Root: If this is part of a subfolder, you can add that here. But generally it's for the main site page, so you can simply leave it empty.
After creating the subdomain, there will be a new folder in your web files with the newly created subdomain name. Please note that for SEO purposes, a subdomain is looked upon as a completely new website and different from your main website(so no link juice from your main site). Just something to keep in mind if you're debating between a subdomain and a subdirectory.
