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The -a switch documentation says the following:
-a With the default display, show the state of all sockets; normally sockets used by server processes are not shown. With the routing table display (option -r, as described below), show protocol-cloned routes (routes generated by a RTF_PRCLONING parent route); normally these routes are not shown.
What is meant by "server processes", is it processes that use server/listening sockets?
The flag simply allows the user to look at all processes/sockets used. If not supplied with -a, then the user will not see the state of EVERYTHING on the system.
To put it nicely, it just means "all"
listeningon a socket, that is, waiting for connections. Without this switch,netstatshows only connected sockets. – AlexP Nov 28 '17 at 19:40