(noun) - Emphasis is a principle of art which occurs any time an element of a piece is given dominance by the artist. In other words, the artist makes part of the work stand out, in order to draw the viewer's eye there first.
Frequently, emphasis is achieved by means of contrast. Obvious contrasting elements create focal points, meaning: places to which one's attention cannot help but be drawn.
All worthy works of art employ emphasis for, lacking this principle, a piece seems monotonous and boring to the eye.