This website is part of gwoptics.org, which was developed by LIGO scientists in Birmingham, UK. You will only be able to run this activity using Firefox browser, and you must first ensure that you have the updated Java plugin, See the instructions linked above in Section 2) for how to update the Java on your local machine (either Windows or Mac).
Try to explore the way the output signal from the device depends on the relative lengths of its arms. Try to make the output signals (there are two of them in the applet) go to zero. Are the conditions needed to make each signal vanish the same? Try to explain what is happening as each signal goes to zero.