Plesk is a software that gets installed on a server and takes over many of the administrative tasks that are otherwise done by hand. The future Plesk server should be freshly set up with nothing on it except the operating system, so your site would not be on it to begin with.
After the Plesk installation is complete, your site will be transferred to the server and hosted there. Plesk's graphical interface would not interfere with your site, e.g. your site would be reachable on example.com and Plesk's interface would be reachable on example.com:8443.
To use an example: you have a server and your site is hosted on it, than you decide to add e.g. a forum software to your site and for this new forum you'd use a subdomain. And that could be so, forum would simply reside on a subdomain. But in case of installing Plesk, it doesn't happen this way. Instead, Plesk takes over your entire server. So from than on, you'd have a Plesk server and your site would be hosted on it, along with any subdomains or other sites.
Hope this explains it.