If you use manual mode for your flash, you will need to change the zoom level yourself. If you are using a zoom lens, as you zoom in and out with your lens the zoom on your flash will change to match it. When you have your flash in TTL mode, which is essentially an automatic mode for your flash, the zoom will automatically change depending on the focal length of your lens. The idea, of course, is that you can channel the light a little bit more when you are shooting something/someone far away (in which case you would set it at a higher number) or you can make the light more disperse to cover a wider area (lower number). With the flash on, go ahead and change the zoom setting while watching the head. Conversely, when you set the zoom toward a large number, it recesses within the flash unit so that the light is channeled out in more of a straight line. When you use wide angles, the head is near the top of the flash so that the light is free to come out in every direction. The head actually moves up and down within the flash unit as you change the amount of zoom.

The zoom setting on your flash sets how disperse or narrow the light coming out of the flash head is.

Zoom flash toys