The way the light hits the leaf is the most interesting to me but I also think the eye is kinda drawn to the lens flare.
The most obvious compositional element is narrow depth of field. another is that the picture is macro so you can see the detail in the leaf. Lastly there is pattern because the the lines in the leaf make a pattern.
If i could chamge one thing i would get rid of the stud in the bench that is visible in the lower right corner. I just think it is not adding anything to the picture so it might as well not be there at all.

Aperture- Aperture is the amount of light that the camera lets in. The lower the f-stop the more light will be let in and the opposite for higher f-stop. This links up with exposure because if you have the aperture too low your picture might be over exposed because too much light came in or if aperture is too high the picture might be under exposed. It also relates to depth of field because the lower the f-stop the more shallow the depth of field and the higher the f-stop the deeper the depth of field.