Like the telephone system, every device attached to the Internet has a unique number, its IP address. Also like the telephone system there is a directory enquiries service to help you find those numbers, the DNS.
If you have someone's name and address you can call a directory enquiries service, give them the details you know and they will (usually) give you the telephone number to call them.
Likewise, if you know a server's host name (maybe www.gondolin.org.uk) you can give that name to a DNS resolver and it will give you the IP address of that server.