It means that your one Vectorworks serial number is being used by another installation of Vectorworks on another computer (which is contrary to the terms of the Vectorworks License Agreement.)
For further explanation about the End User License agreement, please see this article.
So, if you have installed VW on two computers, to solve the concurrent-use clash, please make sure you quit Vectorworks on one computer before you begin using it on the other computer.
If you do this (and are 100% certain that your Vectorworks license is only running on only one computer) and you are repeatedly getting the same message, let us know and we can look into it for you.