Contrast is the difference between the darkest parts of the scene, the shadows, and the brightest parts, the highlights.
For example, a foggy scene is low contrast.
There's little difference between the brightest and darkest parts of the scene.
On the other hand, a backlighted portrait on a sunny day is high contrast.
There's a large difference between the face in shadow and the bright background.
With our eyes, we don't often consciously see these scenes as being low or high contrast.
But, sensors and photography paper do.