What does “Single Site” license mean?

A “Single Site” license lets you use the plugin on one WordPress site, like example.com. It only works on that single installation. You can’t use the same license on subdomains (e.g., site1.example.com) or subdirectories (e.g., example.com/site1). For each separate site or subdomain, you’ll need a separate license, or you can choose an Unlimited license.