What does the word “rolling” mean in “rolling grassland”?

Question

I found the following definition of "prairie" online:

An extensive area of flat or rolling grassland, especially the large plain of central North America.

I understand what "flat grassland" means, but what does "rolling grassland" refer to?

I couldn't find my answer using Google. I do get different sets of images for each search phrase in Google Images, but can't figure out the difference between the two.

Answer 1

It means grassland that covers undulating foothills or bluffs, such as may be seen in Nebraska and South Dakota, and in Colorado and Wyoming on the plains side of the Rockies.

It looks like this: