Ansel Adams and Minor White (among others) preached the idea of visualization; "the ability to anticipate a finished image before making the exposure." In the case of the zone system for exposure and development of film this had much to do with placement of exposure of and subsequent development of the negative to gain desired tonal values.
Today with digital the concept is slightly different and the photographer has more options for post exposure decision making. Whether or not you visualize the finished image, the idea is that the image is more important than reality. With your digital editing program you can easily modify the finished image to look vastly different than the subject in color, tone, mood, sharpness, and virtually all other aspects. This means that you can take what was in front of the camera and make it into whatever you want it to be as an image. This is freedom of expression, but also a challenge put before you to make the image more than it was. You truly need to see, not what is, but what may be.
Adams called conventional photographic recording (what comes out of the camera) acceptable but uninspired. The idea is that the photographer can take the image and put into it an expression that goes beyond the reality. In today's terms, you can take an image into the computer and make it more clearly an expression of your own vision. What is "correct" is not what is important, but what the image ultimately provides the viewer should be what the photographer intends it to be. As a viewer, therefore, we must accept that the photographer is in control of the image, and if the image does not communicate with the viewer either the photographer has failed to communicate, or the viewer does not speak the same visual language.
When we learn to read we learn words, and then sentences, and concepts of the language. The more we read, the greater our understanding of concepts, more complex words, and eventually constructs like metaphors that tell us more than words alone. Visually, the same concepts apply. The more we experience photography of various forms and levels of expression the more we understand how the image can imply mood, direct our attention, or compell us to better grasp what the photographer intends the image to say to us. Leading lines, complex compositions, and abstract foundations of images are all part of what we see but do not always fully understand. More visual experience leads us to better understanding.
We often do not see what it is in our images that is not necessarily wrong, but may be weak, or interferes with the expression we intend. We know we "like the picture" but do not see what it is in the image that explains why. Or, we don't like the picture, but are unable to express why we are not satisfied. Often what others say about the image can help us with the experience as we are directed to different ways of seeing the image. Someone with a greater visual experience will often point out aspects of the image that never occured to us. A few modifications to cropping, tonal controls or simplification of the image in some way can turn a good image into a great image and most of those modifications are simple, but rarely intuitive.
All of this boils down to a few basic ideas. We have to look at images, more images, many images. We need to look at images we do not necessarily like as well as those that instantly inspire us. We need to study those images - not for the subject matter but for the structure. The structure is the abstract foundation on which the image is built, and the structure of the color, tone and saturation is what leads us to look in certain places - that along with composition leads our eyes to where the photographer wants us to look. If we have a strong visual experience we will be more apt to discover the weaknesses in an image and then learn how to modify an image to correct those weaknesses. Ultimately, we will learn how to better express ourselves, and eventually instead of just showing pictures of what we saw, we will create visual expressions of our experiences, and share them with others.
"Art is not what you see, but what you make others see." – Edgar Degas