I can go and explain this but it's far easier to go outside yourself and see it.
Have someone stand in front of you in a bright sunny day. Look at the person's face. Notice how nose casts shadow on his/her face. See how much forehead reflects the light. Also notice, the darkness/shadow the chin makes on his/her neck. Make a mental note of the difference in brightness in these area. Or better yet, take a photograph of this.
Then do the same in bright but cloudy day. Then do the same in heavily clouded day. Or, do it in shaded area.... not directly lit.
OR... if you must do this with landscape, pick a subject, pick a camera position and do the same.
You'll notice, more clouded the sky is, the difference gets smaller and smaller.... Remember, the difference of the light level between bright and dark IS the contrast.