Frames of Mind are a part of the weather system in RGB's world, similar to clouds, but not quite the same. They are made of Stuff, much like trees, and are subject to changing form. They can fill in and form platforms, align into thought patterns, and move. They can be traversed if need be.
Frames of Mind are first introduced in Chapter 5 of The Property of Hate, in the Plains of Hesitation. There, the Frames are more fleeting, the hesitation causing them to crash into the ground and evaporate at any given moment.