Well, NASA tells us that, "A black hole is a place in space where gravity pulls so much that even light cannot get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying." (Ref.NASA.gov feature page)

A "White Hole" in theoretical relativity is, and this is my simple explanation, a point in space that cannot be entered from outside but it is a portal where light and objects may emerge. It is, theoretically, a hole in the fabric of space where matter is either created or passes through but not into.

Since you asked specifically "What is a white whole?" I am going with this answer, "White whole-wheat flour is flour milled from hard white spring wheat, rather than traditional red wheat. In the United Kingdom, whole-wheat flour is more commonly made from white wheat instead of red as in the United States. (Ref. Betty Crocker)

I hope that helps! Now, I need to get back to my observatory…I’m watching for holes in space-time. I have a plan to travel back in time and keep the Jonas Brothers from breaking up.