Emphasis
The deliberate creation of one dominant point of visual attention within a composition.
In practice
Emphasis makes one element the clear first thing people see, the focal point. It comes from contrast, size, color, isolation or position. A sale flyer that makes the discount, the headline and the photo all equally loud has no emphasis and reads as noise; enlarging only the discount and quieting the rest gives the eye a place to land. Two competing focal points tend to cancel each other out.
Often confused with Visual Hierarchy
Emphasis creates one dominant point; visual hierarchy orders all the elements, first, second, third, through the whole composition.
Try this
Mark the one thing on a busy flyer that should be seen first, then redesign it so only that stands out.