A white hole is the hypothetical "exit" to a black hole.  Black holes are the remains of dead stars that have imploded upon themselves.  The material is so dense, it generates a tremendous gravitational attraction.  This attraction is so strong, note even light may escape, hence, the term "black hole".  A white hole, hypothetically speaking, is the exact opposite, more like an exit force from the black hole, repelling all matter and light, allowing passage of such from within, but allowing no entry of such from the outside.  This line of thinking hails from Einsteins theory of general relativity, relating the existence of parallel universes, wormholes connecting the parallel universes, and exceeding the speed of light, so as to turn back time.  It is said Einstein developed his theory of general relativity by watching a clock recede from view as he was riding on a bus, analyzing what would happen if he could accelerate the bus past the speed of light.