Clearing the mind and having a single focus zone sound like it should be simple, but it can be so difficult. As a musician, I've experienced it many times, but cannot achieve it every time I play or perform. But, I know the joy of the experience of performing when the mind is strictly focused on the music. In those moments, you are no longer aware of the audience, the visual image of the performers around you, the extraneous sounds in a concert hall, etc.
I've read of this type of focus being shared by athletes such as the pitcher on the mound filtering out everything except the process of pitching the ball. Then there are those that have reported that when "in the zone", the ball is moving slower and you can see everything clearly.
Most people have experienced this clarity of focus in some aspect of their life. The challenge is achieving it when desired.
Applied to photography, my interpretation would be the ability to focus on what we see without any filters or biases. It's the art of seeing that which is all around us but capturing that unique image. However, most of us usually have a preconceived goal in our photography. We take our camera and want to capture wildlife, or people on the street, or particular sites we want to preserve as part of trip, etc. And, quite often, I know I've thought to myself "well, nothing of interest out here today!" when I've headed out with the hopes of getting some wildlife shots. I think it is at those times that we can learn to pursue photography in a different mode, one in which we just explore the environment with our eyes and seek to see it more clearly.
Or, we need to deliberately practice this type of photography experience on a regular basis....filter out as much as possible, look with our eyes, and capture what is normally blocked by our other senses and over active brains.