Haha! I will show my age and give you the meaning of "presets" before cameras even had batteries.
An "automatic" lens will remain at the maximum aperture permitted by that lens until the last split second when you actually take the picture. There is an interlink between the body and the lens that causes the lens to "stop down" to the required aperture as you take the picture. This used to be a mechanical interlink and you could actually push a little pin or slide a protruding piece of metal on the back of the lens and check out the diaphragm action. Now it's controlled by electrical contacts.
In the olden days, there were no such interlinks. You could "preset" the aperture on your lens to, say f/8, and it would "remember" where you set it. After you set it for the proper exposure, you would open the lens manually to the maximum aperture and touch up your focus. Just before taking the picture, you would rotate the aperture ring around until you bumped into the preset and this would tell you by tactile sense that you had re-set the lens at the proper preset aperture and then you could take the picture.
Imagine that. Although it took a few seconds to do this, it was a darn sight faster than counting clicks from the maximum aperture to the f-stop that you meant to use.
Why bother with this? It is easier to focus accurately when there is more light coming into the viewfinder and it is easier to focus on the main subject when you use the maximum aperture, because the limited depth of field will make any discrepancies more obvious.