Remote login means to "log in" to a remote computer (or, to use the
historical term, a "host") as though it were local.
The idea of "remote login", as opposed to "local login" is
significant in the history of computing -- originally, a user
"logged in" at a basic display terminal which was directly
connected to a hardware port on a multi-user host computer. When
the Internet's predecessor ARPANET was being developed, this was
viewed as its likely main application.
If the host was connected to a network, remote login allowed
users to log in to other networked hosts over the network as though
their terminal was directly connected. Either way, the idea of
"logging in" is still based on getting a command line
shell on the target system. On our Unix systems, we
nowadays talk about a shell window, which performs
this function.
Some systems do not (even now) support remote login, in most
cases because they don't support a decent "command-line"
interface.
Different operating systems have (or used to have... ) quite
different procedures for handling local logins, making the problem
of providing a generic remote login facility (potentially) quite
complicated.