I got a nice explanation in another forum that I wanted to share:
Any Microsoft network has a computer known as the "master browser". In a domain, this would normally be one of the domain controller, but in a workgroup, it works a little different.
Usually, the computer that is on first (meaning, when it boots up, it can find no other computer in its workgroup) will be the master browser. The next computer on the network will then automatically detect the master browser, and be happy with that.
The issue that you are having is that a computer that is the master browser are detecting another computer claiming to be the master browser. This could be because your wireless network was disconnected for a brief period of time. If that occurs, both your desktop and wireless computer will set itself as the master browser (because they are now on seperate networks, and the only computer on that network).
You don't really have to worry about this error, it's something that your computers are working out between themselves, so there's nothing that you have to do to resolve the issue.