A license is a way to protect your intellectual property on the map.
For example if you choose "All Right Reserved", no one has the right to edit, redistribute, ... your map. If you are totally open you can put it in the "Public Domain". You can Google for the name of the licenses and you will get more informations on all the things you can / can't do with it.
Anyway, if you just want to create a map and post it. I don't really know about all that stuff myself, I personnaly put all my things in Public Domain so people can do anything they want with my maps :p
I'm pretty sure there are people in here that know more and will tell you which one are the best :P
It has nothing to do with the map making process, we implemented it because we want to make sure that people who upload their projects on mapster got legal possibilities in case of conflict (map stealing, ...). I'm not aware of anyone that sued someone else for this matter but just keep in mind that you are protected when submitting the map.