It has the same (technical) owner as the main domain. An example of a subdomain is, where is the main domain and is the subdomain. The website being checked is a subdomain. Only if you never have to enter data (like logging in or filling in a form) an SSL certificate is not really essential. If there is no SSL certificate communicate is not safe. An SSL certificate secures communication between your computer and the website.
We therefor recommend you to be cautious before you enter any personal data. What you see may actually be located on a completely different website.
This means that the website is including or iframing functionality located on another webserver. This website is a website within a website.