Imagine that your viewfinder is divided into thirds both vertically and horizontally. Into 9 pieces.
Now place your point of interest in the intersecting lines. The theory is that your photo becomes more balanced and it is more natural for the viewer to interact with the image.
Studies have shown that when viewing images that people’s eyes usually go to one of the intersection points most naturally rather than the center of the shot – using the rule of thirds works with this natural way of viewing an image rather than working against it.
Take a look at some of your favorite photographs or pieces of art. Can you find the rule of thirds being applied?
Take a look at your photographs. Can you see where cropping them would make a difference.
Once you learn the rule, don’t forget to go and break it.
Check out https://digital-photography-school.com/rule-of-thirds/ for more examples of The Rule of Thirds.