DoS is not Disk Operating System :). This stands for Denial of Service attack and briefly described is when to many calls are made to your service so it stops responding to intented users/customers. for more info see Wikipedia.
MS MVP Omar AL Zabir wrote good blog entry about how to prevent DoS attacks on ASP.NET sites and expecially web services.
When you make good ASP.NET application and clear most of the bugs (yep nobody is perfect) it is time to adjust it to be high available and fast respondible Prevent Denial of Service (DOS) attacks in your web application is good start to protect your creation from so called hackers attacks :)