Contrast is defined as “the difference in visual properties that makes an object distinguishable from other objects and the background”
(source: wiki : https://en.wikipedia.org/wiki/Contrast_%28vision%29 ). We like to say it means to make some things stand out more than others. A website needs to have some variation and contrast.
In this example, you see the background has a subtle yet powerful image. However, the foreground or informational panels almost jump off the page and catch your attention.