what does this mean?

2003-12-20 14:43:11 StandardContext[/balancer]Exception starting filter BalancerFilter
java.lang.NoClassDefFoundError: org/apache/commons/digester/Digester
at org.apache.webapp.balancer.RulesParser.createDigester (RulesParser.java:107)
at org.apache.webapp.balancer.RulesParser.<init>(RulesParser.java:85)
at org.apache.webapp.balancer.BalancerFilter.init (BalancerFilter.java:121)
at org.apache.catalina.core.ApplicationFilterConfig.getFilter (ApplicationFilterConfig.java:272)
at org.apache.catalina.core.ApplicationFilterConfig.setFilterDef (ApplicationFilterConfig.java:355)
at org.apache.catalina.core.ApplicationFilterConfig.<init>(ApplicationFilterConfig.java:126)
at org.apache.catalina.core.StandardContext.filterStart (StandardContext.java:3646)
at org.apache.catalina.core.StandardContext.start (StandardContext.java:4263)
at org.apache.catalina.core.ContainerBase.addChildInternal (ContainerBase.java:866)
at org.apache.catalina.core.ContainerBase.addChild (ContainerBase.java:850)
at org.apache.catalina.core.StandardHost.addChild (StandardHost.java:633)
at org.apache.catalina.core.StandardHostDeployer.install (StandardHostDeployer.java:316)
at org.apache.catalina.core.StandardHost.install (StandardHost.java:859)
at org.apache.catalina.startup.HostConfig.deployDirectories (HostConfig.java:723)
at org.apache.catalina.startup.HostConfig.deployApps (HostConfig.java:473)
at org.apache.catalina.startup.HostConfig.start (HostConfig.java:1002)
at org.apache.catalina.startup.HostConfig.lifecycleEvent (HostConfig.java:393)
at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent (LifecycleSupport.java:166)
at org.apache.catalina.core.ContainerBase.start (ContainerBase.java:1133)
at org.apache.catalina.core.StandardHost.start (StandardHost.java:816)
at org.apache.catalina.core.ContainerBase.start (ContainerBase.java:1125)
at org.apache.catalina.core.StandardEngine.start (StandardEngine.java:518)
at org.apache.catalina.core.StandardService.start (StandardService.java:519)
at org.apache.catalina.core.StandardServer.start (StandardServer.java:2343)
at org.apache.catalina.startup.Catalina.start (Catalina.java:581)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke (Method.java:324)
at org.apache.catalina.startup.Bootstrap.start (Bootstrap.java:297)
at org.apache.catalina.startup.Bootstrap.main (Bootstrap.java:398)