Impossible Figures are optical illusions created by a 2D representation of a 3D shape.
Our brains want to construct a 3D shape from this 2D representation; however, our brain is most familiar with objects in the real world. It uses that information of everything we see everyday to extrapolate what we would think of the most logical construction of the 2D representation.
Depending on how we follow the form of one of these figures, we eventually reach a point where the shape of the form from oneside meets the other. At this point though we reach an inconsistency where it would have been impossible for the shape to take that form in the real world. This is an impossible figure.