This button is used to both initialise and park the telescope. When first used, to initialise, the brakes are released and both axes begin to move slowly. This is done in order to determine the absolute position of the telescope. This normally takes no longer than thirty seconds. A popup will inform you of success or failure.
If successful, it is important to check the validity of telescope Az/El coordinates as reported on the main window. This is necessary because it is possible for the telescope to obtain incorrect coordinates and it's better to spot this early on. (Note, if started from the Home position, coordinates should be (roughly) 103 azimuth, 93 elevation.)
If initialisation fails, simply press the initialise button again, and repeat until successful (failure generally means that encoders are dirty, usually after some bad weather - see appendix A). After three unsuccessful attempts it may be best to move the telescope using the service program (see appendix B), and then run the turret program again.