The crown of a watch is used for winding the mechanical watch and setting the time and/or date. In most cases the crown is positioned on the right side of the watch, usually at the 3 o'clock position. There are also watches where the crown is on the left side of the watch. (very useful for the left-handed people)
Some watches, especially dive watches, have a screw-down crown. In this case, the crown can be tightened to prevent water from entering the case or accidentally adjusting the crown. In order to adjust time or date, your screw-down crown must be unscrewed before you can pull it to its first or second position. And you will notice this as it springs open if fully unscrewed.
There are crowns in different materials and shapes, just like you will see in the selection below.