This command "jupiters" a server. That is, a fake server is created by services and connected to the network in place of the real one. This is mostly useful in the case of a server repeatedly splitting and rejoining the network, servers which are suspected of being compromised, or some other server failure(s) affecting general network performance. A jupitered server can be removed from the network with a squit command.
This command accepts 1 or 2 optional arguments, the server address to be jupitered, and the optional reason for the jupiter. If no reason is specified, then your name will appear as the reason for the server being jupitered, appropriately date stamped. You should always specify the reason for the jupiter however, to prevent someone from removing it prematurely. To jupiter badserver.ravenchat.net for the reason "Frequent netsplits", you can use this command:
/msg OperServ jupe badserver.ravenchat.net Frequent netsplits
The jupiter should be removed either when the server problems have been fixed or it has been determined that the server will no longer be permitted to remain linked to the network.