A "flipping" optical illusion is an illusion that can appear in two different ways, so the image "flips" in your mind. Examples are the Necker Cube or the following ASCII illustration attemts:

 ___/___   /
 \ /    \ /
 /_\____/ \
Are the longer fins pointing inwards or outwards from the screen?

 /  /|
/__/ |____
   | /   /
Are these stairs on the floor or in the ceiling?

If you stare at such a 2D image, your mind makes up a 3D representation of it. The problem is that the figure is ambigous; in 3 dimensions, it can be interpreted in two different ways. These two interpretations are the ones your mind flips between.

A similar phenomenon (but not related to 2D/3D) is the Pretty Woman/Old Hag imgage