This does not mean that the Nationals have lost these players for good. It means quite the opposite. It simply means that all these players have completed their original contracts but cannot come to an agreement with their club on a new contract. Both sides will now make a case to an arbitrator on a size and length for a deal. The arbitrator will then decide which side presents the better deal. (Definition from livestrong.com)
These players are all key players for the Nationals franchise. The club just recently acquired the services of Gio Gonzalez in a trade with the Oakland Athletics.
A reason why the two sides may not be able to come to an agreement could be because of the Nationals apparent interest in Prince Fielder. Take this for what you will, but the Nationals appear to be pursuing Fielder and he will not come at a cheap price. They could be trying to low-ball these free agents in order to make sure that there is enough cap room in case they decide to try to sign Prince Fielder.
This is all still just speculation since the Nationals have been private about their thoughts on signing the high priced free agent first baseman. It would be an excellent idea, however, to sign the six players that have filed for arbitration. They will be able to contribute to the organization for years to come.