Websites are installed on web servers on normal computers, as we said before. These normal computers have IP addresses and, using the IP address, we can access our target website. Now, in many scenarios, our target website, or our target server, will contain a large number of websites, hence it'll have the website that we are targeting, but it will also contain other websites on the same server, hence on the same filesystem. For example, if we could not find any vulnerabilities in our target website, we can still try to hack into any other website that is installed on the same server. If we can do that, then we will be able to gain access to the server. Gaining access to the server basically means that we have access to all the other websites, because the server is just a computer, and we can navigate to the website that we want to hack and gain access to that website. Suppose we are trying to hack into a website and we can't find an exploit, then the next step will be trying to hack any other website that existing on the same server. Hence, what we mean by exist on the same server is they have the same IP address.