Some scripts/programs may ask you for your web root (or document root) when performing the initial setup. This is the base of your website, from the point-of-view of the server, not the client/browser.
For all our newer servers, this is /var/www/vhosts/(domain)/httpdocs/, where (domain) is your domain name. For example, for example.com it would be:
/var/www/vhosts/example.com/httpdocs/
Note that www. is not part of the path.
However, if you are on our older server alpha.jabservers.co.uk then the path is /home/httpd/vhosts/(domain)/httpdocs/.
Also, if you are using a secure website, and have not enabled the single directory option in Plesk, then you will need to replace httpdocs/ with httpsdocs/ to point to the secure site files.