Remote login means to "log in"[2] 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, all "logins"
were local -- a user "logged in" at a basic command-line text
display terminal which was directly connected to a hardware port on
a multi-user host computer -- what we would nowadays call a
"mainframe". 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 to
the remote machine. 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
to the operating system.
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
complex.
[2] There's no
universal agreement on whether we should use the words "log in",
"login" or even "logon". Wikipedia, for example, considers
them as equivalent.