The hi-hats are a pair of cymbals mounted on a stand, one upside down and the other the right way up, both held in place on a central spike and connected to a footpedal at the base of the stand. By operating the pedal, the drummer brings the hi-hats together. If you watch a jazz drummer play, you’ll see that the hi-hats spend most of the time closed, but open and shut quickly on the second and fourth beats of the bar. In just about all other modern styles the hi-hats are kept closed or half-closed and played with sticks, anchoring the drum track by beating out eighth-notes.
The role of the hats is to give life to the kick and snare pattern. Acoustic hats vary from small, thin and bright (jazz, pop, dance) to big, thick and dark (rock), and only really have four sounds: closed, half-closed, open and pedalled. Of course, each of these has endless tonal variations, as a browse through any good drum sample library will illustrate.