This is the part of an instrument’s frets that protrudes above the fingerboard. The fret is secured into the fingerboard with the tang, which is pressed or glued into a slot in the wood (or whatever the fingerboard is made of). The crown is the actual part of the fret you see, and that you press the strings against to sound notes. Various crown sizes are available, which can radically change the feel and response of the instrument, and may even change the tone to a certain extent.
Fret crowning is the process of using files and other tools to reshape the frets, especially after they have been leveled or dressed by a tech or a partial or total refret has been performed.