This command will remove an akill from the server you are connected to. Unlike the rakill command, which will remove the akill from all the servers, the unakill command will remove it only from your server.
Example:
/unakill mark@*.webmaster.com
This can be a way to give someone a chance to come back onto the network while still limiting their use and keeping a close eye on them. But generally you will want to remove akills globally rather than locally.