A New Site is a site that has no existing content. On new sites, the demo content will be imported for you during theme installation.
An Existing Site is an established website with existing content that you have published: posts, pages, images, etc. Sites with existing content require a lot of extra time and care during theme installation.
For existing sites, please do the following prior to installation:
Make a backup of your website
Update your website to the latest version of WordPress
Do a plugin audit
Upgrade to PHP 7.4
Exception: If you tried to install your theme on your own and imported the demo content, your site is still considered a new site. Your site is only considered an existing site if you have your OWN content published.