Imaging a rectangle canvas as an image area. If landscape (horizontal), draw two vertical lines, one 1/3 of the way in from the left, and the other 1/3 of the way in from the right. If portrait (vertical) draw your lines one 1/3 of the way from the bottom and one 1/3 way down from the top.
Most art experts, teachers, and serious students of the Masters feel that where these lines intersect are the points of greatest interest to the viewer’s eye. Some also call it the ‘point of focus’.
Personally I’m not much for rules, I’d rather just trust my gut, but don’t quote me on that. Textures and patterns for instance completely blow this concept out of the water in that they do not have single focus points.