There was a time when lenses were fully manual. It's not only about focus. Aperture size and control also. To take a shot, you would set aperture to widest first, focus and compose, stop down to desired aperture size, then finally press the shutter release button.
So why set to widest first? You can't focus using a small hole! You just can't see through it.
People eventually got tired of constantly going to widest then stopping down. They made the preset lens which had two aperture rings. One for setting the aperture size and one for stopping down and opening wide quickly. It was a huge improvement from fully manual lenses but there had to be an even easier way.
That's when they devised a way to keep the lens aperture wide open all the time. It will only stop down when you press the shutter button. The lens went back to having just one aperture ring. Aperture size is set a fraction of a second before shutter curtain opens. These lenses were called Automatic lenses. There were some built with an Auto/Manual switch to make the lens compatible with older cameras that did not have the mechanism needed to keep the lens wide open all the time.
To go further, linkages were added to the camera and lens so that the camera was able to track what aperture you would set it even though the aperture is actually wide open. When you press the shutter release button, the lens will stop down correctly to what you set it to right before the curtain opens. Lenses aren't called Automatic by this time since all of them already are. This is now how all dSLR's work. Set your camera to 2 seconds and let if face you so you can see how it happens.
Now you know!