Question

what are white holes?

Answer 1

White holes  is the reverse of  black holes. White holes is a region of spacetime, that can't be entered from outside, but light  and matter can escape from it.
In the case of black hole , it is possible to enter from outside , but light and matter can't escape from it.

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Answer 2

In general relativity, a white hole is a hypothetical region of spacetime and singularity which cannot be entered from the outside, although energy-matter and light can escape from it. In this sense, it is the reverse of a black hole, which can be entered only from the outside and from which energy-matter and light cannot escape. White holes appear in the theory of eternal black holes. In addition to a black hole region in the future, such a solution of the Einstein field equations has a white hole region in its past. However, some believe this region does not exist for black holes that have formed through gravitational collapse, nor are there any known physical processes through which a white hole could be formed. Although information and evidence regarding white holes remains inconclusive, the 2006 GRB 060614 has been proposed as the first documented observance of a white hole.

Supermassive black holes (SBHs) are theoretically predicted to be at the center of every galaxy and that possibly, a galaxy cannot form without one. It has been proposed by Stephen Hawking that these SBHs spawn a supermassive white hole.