The @doing command allows you to tell other players what you are 'doing'. Your
text shows up on the rightmost column next to your name on the output of most WHO commands.
Despite the name @doing, most often the command is used either in response to a poll or is set in a player's @aconnect as something humorous, and doesn't generally relate to what a player is doing. However, historical usage was to put D: <activity> if you did want to let people know what it was you were up to.