What does this in the MOTD for an MC server mean?
What it does is allows certain websites like minecraft forums and stuff like that recognise that it is your server. You are generally given a MOTD that is unique when you try and register your server. IF it is your server, you are able to easily change the MOTD temporarily and allow the site you are registering your server with to verify that it is actually yours. Once the activation completed, you can change it back to what ever you like.