In most dictionaries, contrast is defined as an obvious difference between two or more things. It is slightly the same meaning in art but with it denotes the difference between light and dark colors or areas. Contrast destroys the harmony or unity of art to emphasize an important object or strong emotion.
Imagine a painting of a garden with so many bushes, grass, and trees. They are all green in general right, then you can see at the center a bright red swing. The contrast of the background’s color (green) and the subject (red) makes the swing pop out and can make the viewers think of the story behind it.