For some time, I've known that it was possible to change the description of a particular server advertised on the Metaserver under the "Comment" column of the Network staging screen.
However, despite repeated efforts to find the appropriate command on the Freeciv wikia site (including searches for "metaserver info" and the like) I just couldn't find the info I needed.
Then today I noticed someone had changed the default message to something else. I joined the server and saw that the "metamessage" command was used. Cool!
Here's the help section found on the server for the command in question. I found it was pretty useful. I wanted to set up a quick and dirty server with the description "Just Practicing" ... so I used the command "metamessage Just Practicing" and it worked.
/help: Command: metamessage - Set metaserver info line.
[16:21:52] /help: Synopsis: metamessage <meta-line>
[16:21:52] /help: Level: ctrl
[16:21:52] /help: Description:
[16:21:52] /help: Set user defined metaserver info line. If parameter is omitted,
[16:21:52] /help: previously set metamessage will be removed. For most of the time user
[16:21:52] /help: defined metamessage will be used instead of automatically generated
[16:21:52] /help: messages, if it is available.