The kernel monitor is a servlet that collects information on the running of your site. You should be able to find the monitor via the URL http://localhost:8080/webMathematica/KernelMonitor. (You may have some different URL for accessing your server.) Upon access, the monitor brings up a page showing the current status of webMathematica, describing various parameters of the site, and giving status information for each kernel. If you look at this page, access some JSPs, and then look at the page again, you should see updates, such as a change in the number of times kernels have been accessed.
The section of the monitor that displays kernel status shows useful information that can help you study the performance of your site, including how much time each kernel has been running and how much time each kernel has spent servicing requests. It also shows the ratio of the two. If the pool has more than one kernel, it also shows averages of all kernels. You can monitor the load on the site by studying the ratio of time spent servicing requests and use the information to tune its performance.
The page provides a number of controls: at the top are buttons that cause all kernels to restart, the monitor to reload, and any logs that have been collected in the servlet to be cleared. There is a button to turn verbose logging on and off and a button that includes the entire log file in the output of the monitor.
Finally, there are two buttons that allow the user to capture and then display input to Mathematica that can be used for debugging purposes. The section on Debugging MSPs describes the operation of these buttons.
For security purposes it would be sensible to restrict access to the kernel monitor. If the servlet engine is accessed via an Apache web server, access can be restricted in the server configuration files. The section on Apache and Tomcat describes how this can be done.