A key function of the server is to listens to requests over a network and returns the requested files, data, etc. Linux machines have servers installed for many different protocols: Eg. HTTP Server, SSH Server, FTP Server, etc. The computers which request data from a server (client machines) use either a web browser for HTTP requests or an SSH or FTP Client and can be identified by their individual, unique IP addresses.
Linux admins used to rely on netstat to review connections, but the faster, more reliable ss has replaced it.