A deployment descriptor (likely web.xml in your case, since I think you posted an earlier question about building a webapp) tells the application container how the web app should be configured. This is where you register your servlets and filters, add context parameters, and lots of other useful things.
Here is a link that describes the various parameters and their uses: http://download.oracle.com/docs/cd/E13222_01/wls/docs81/webapp/web_xml.html
If you read that it is optional in NetBeans, it may be the case that NetBeans provides a default for you (maybe it lets you register your servlets through the IDE). Still, it's good to know what goes on in web.xml in case you ever build a webapp without using NetBeans, or need a lower level of configuration than NetBeans provides.