A histogram is a graphical representation of the pixels exposed in your image. The left side of the graph represents the blacks or shadows, the right side represents the highlights or bright areas and the middle section is mid-tones (middle or 18% grey). How high the peaks reach represent the number of pixels in that particular tone. Each tone from 0-255 (o being black and 255 being white) is one pixel wide on the graph, so imagine the histogram as a bar graph all squished together with no spaces between each bar.
My histogram is kind of shifted in left side which indicates the details in photograph are poor. Maybe some curves can be applied on Photoshop to lower the black and higher the whites. The alley is fully on light but other than that the trees throw shadows on the grass and trees themselves have no detail. The mid tones don’t exist.