That indicates that this servlet should be loaded (i.e., instantiated and its init method called) at the startup of the web app. Normally a servlet is not instantiated until it is first called. But if the overhead of the instantiation is substantial, that would delay its reponse to the first request. So load-on-startup allows you to indicate that it should be loaded right when the web app is started.
The "2" indicates that this particular servlet should be the second one to load - there is probably another servlet with a load-on-startup with a "1"; that is loaded first, and then so on in ascending order.
load−on−startup element that tells the JSP/servlet container that we want this servlet to be preloaded when the Web application starts. You must pre−load the ActionServlet, or your Struts Views will not load all of their necessary resources.