"Not a good place, ironically, to hesitate." -RGB
The Plains of Hesitation are a location within RGB's world, consisting of red-orange grass, blue sky, and Frames of Mind. Trees rarely grow there, allowing greater populations of creatures such as Fears, Doubts, and Griefs to lurk and gather in the tall grass.  The Plains were introduced as a location in the last panel of Page 87, in Chapter 5, and were featured as the main location of our protagonists through until the end of Chapter 6.
This was the location where RGB and Hero first confronted and argued with one another over the issue of Hero's ability to return home and her identity as a person, nearly destroying their partnership. This is also the location where we are first introduced to Negative, in his namesake chapter. The vines left behind by the transformation are still there, as found by the re-inspired Idea.