write

a message on someone's terminal

in the terminal: help write

Send a message straight to someone's terminal, if they're logged in:

write alice hello, want to try the board later?
write alice              then type, and ctrl-d to send

finger @runv.club shows who is on. If messages pop up while you're working and you'd rather they didn't:

mesg n      nobody can write to you
mesg y      back on
mesg        tells you which one it is

talk isn't here: Debian dropped it. For a real conversation, chat.