Based on the description of the picture, it is a standard basketball hoop with a rim. With a typical backboard positioned according to the standards of basketball (not dropping or incorrectly installed), the rim should be at a right angle with the backboard. Thus, the plane containing the backboard and the plane containing the rim should be at right angles to each other. Therefore, the planes are perpendicular to each other and the backboard is perpendicular to the rim and vice versa. For something to be perpendicular to something else, i.e. a line or plane, they have to form a right angle which this does.