Undulating doesn't necessarily mean hills. It means repeated changes in elevation in a landscape, which can often be very small. So you can have lowlands that are undulating. Lowland meaning ground that has a low elevation in relation to ground around it or relative to sea level.

As Chris said. Undulating means that they have a gentle change up and down like gentle waves. Picture them as the surface of a very gently wavy sea. It sounds beautiful : )

"Undulating" is related to the word for "wave" in some other languages--"onda" in Spanish, "onde" in French. It means the landscape is curvy, and has gentle, small, smooth hills and valleys. In the United States, Kansas is flat; New Hampshire is rugged and mountainous; but Wisconsin has a landscape of gently undulating hills. This is a picture. Notice that it is all low ground--it is covered in grass, nothing is very high, there are no snow-capped mountain peaks. http://media-cache-ak0.pinimg.com/originals/5f/28/38/5f28383f3e0ba8132ea33559083b33c6.jpg