A White Hole was a spatial anomaly that was the reverse of a black hole. A black hole sucked time and matter out of the universe, while a white hole returned the same into its vicinity.
After the Dwarfers used an intelligence compression technique to increase Holly's IQ, she discovered that her lifespan had been reduced to minutes. To preserve her run-time, she shut herself off and put Red Dwarf into emergency power mode. The crew had to fend for themselves, scavenging food from the cargo bays.
When Rimmer and Kryten were returning from a food run, they encountered a temporal anomaly. One half of the corridor was operating much faster than the other, causing conversation to be very difficult. They fetched Lister and The Cat and headed for the Science Room, hoping to ask Holly what was happening.
The sensors showed what Kryten guessed was a White Hole, a space phenomenon that returned time and matter to the universe. The random pockets of time that it was throwing out were affecting causality, but those time jumps could be resisted through concentration. Since its engines were dead, Red Dwarf could not escape the white hole's influence.
The white hole glowed with a blinding white light and was surrounded by planets and two stars that had been expelled from its event horizon. There was also a lot of random gases and electromagnetic radiation being thrown out of the white hole.
Turning Holly on for a brief conversation (in which Rimmer described the white hole as "spewing time"), the crew received a computer slug containing the solution she had determined. She proposed that Starbug fire a thermonuclear weapon into the nearest star, causing a solar flare that would push one of the star's planets into the white hole, blocking it up.
Lister disagreed, believing that his pool playing skills perfected at the Aigburth Arms qualified him to make a better attempt. He fired the weapon, causing the planet to collide with a second, which hit a third in a spectacular trick-shot that blocked the white hole.
Eradicating the white hole erased its influence and nullified all the time it had spewed into the universe. This caused everything that had occurred in the days since it had opened to cease to exist. Holly was restored and the timeline reset to normal. (RD: White Hole)
Behind the scenes
White holes are a theoretical possibility and are the reverse of black holes, according to Einsteinian physics.