What is the default value for resource?

[sam]

Hi,
  I am reading
http://maven.apache.org/plugins/maven-war-plugin/examples/adding-filtering-webresources.html,
There
are some configuration like below:
<configuration>
       <webResources>
         <resource>
           <!-- this is relative to the pom.xml directory -->
           <directory>resource2</directory>
          * <!-- the list has a default value of ** -->
*            <includes>
              <include>**/*.jpg</include>
           <includes>
         </resource>
       </webResources>
     </configuration

What does the "dafault value" mean?I can't understand.