It is said to be the exact opposite of a black hole. While a black hole sucks in objects to its singularity, the white hole emits matter from its singularity. A white hole may be formed after a black hole can no longer suck things in. Immediately, it turns into a white hole and starts emitting the things that it has previously sucked in before. Nothing can enter a white hole as its emitting force is too great. In this sense, it is the reverse of a black hole, which can be entered only from the outside and from which energy-matter, light and information cannot escape.
Like black holes, white holes have properties like mass, charge, and angular momentum. They attract matter like any other mass, but objects falling towards a white hole would never actually reach the white hole's event horizon.