There's normally a profle graph that accompanies a route, where the distance travelled extends on the horizontal axis, and either the gradient, or altitude extends on the vertical.
The scales are wildly different, and even a nearly flat ride over a suitable distance will look like grass.
What you or I call 'hills' may vary, but anything under 20 metres in a kilometer (2%) will pass pretty much unnoticed.
Try drawing out a journey you know on a mapping site, then looking at the profile. You'll soon get it.