It is the button on the end of the spindle that is used for adjusting the time and/or date on an analogue (non digital) watch. In older watches and pocket watches which are not powered by batteries it is also used to wind the watch spring to give it power. Generally the winding is done with the crown flush against the watch, pulled out once it adjusts the date (if it has one) and pulled out a second "click" it adjusts the time. The crown is generally round and ridged to provide grip on the finger.