Emphasis: the principle of art that makes one part of a work of art dominant over the other parts.
When you want to remind yourself to do something, you leave yourself a note in a very obvious place. When you want to make your point clear, you raise your voice or speak more slowly. These are examples of how people use emphasis to focus attention on something. Likewise, when you want someone to notice a specific part of an artwork first or you want one part to stand out more than another, you emphasize that part.
Artists use any device they can to give emphasis to a part of their work. They could use color, texture, size, shape, contrast -- anything to draw an observer's eye to a particular part of a work. What is the emphasis in Claude Monet's Impression Sunrise, 1876?