A Black Hole was a spatial and temporal phenomenon caused by the collapse of a suitably massive star. No light or matter could escape from its massive gravitational pull, and it was the most destructive force known in the Universe.
Holly once forced the boys from the Dwarf to evacuate Red Dwarf when she spotted five Black Holes on the scannerscope. After piloting the ship past them, she realised that they were merely 5 specks of grit on the scannerscope. (RD: Marooned)
Dashing test pilot Ace Rimmer once successfully took his light speed ship on a loop-de-loop around the Event Horizon of a Black Hole, and like "a frolicking filly in a harvest-time pasture" around the singularity. (RD: Dimension Jump)
When the Cat rushed into the sleeping quarters and said there was a situation developing, he forced the others to guess what was happening through an impromptu game of charades. A post pod had arrived, but the others first guessed that it was emotion sucking chameleonic monsters, rogue viruses, Black Holes, "death worms" and other calamities. (RD: Dear Dave)
Red Dwarf strayed too close to a black hole after Holly had gone offline. This caused multiple time distortions to occur aboard the ship. Before he deactivated himself, Holly had told Talkie Toaster how to escape from a black hole's gravity well. The Toaster, once placated with having them eat lots of toast, instructed the crew to fly as fast as they could into the hole, loop round the singularity at its centre and use the gravitational pull to fly out even faster.
One of the things they encountered in the black hole was the Omnizone, a gateway between dimensions. The strange gravitational effects inside the black hole caused the crew to become spaghettified (turned into living spaghetti). After returning to real space, they went to rescue Lister from the planet where he had crashed. Owing to the effects of the black hole, Lister had waited 34 years for them while only sixteen days had passed aboard ship.
After picking them up again, Starbug found itself in the wrong dimension. The asteroid belt they encountered was being sucked slowly into a black hole. The GELFs that lived on the asteroids had a plan to terraform a planet which they could ride through the hole and into another universe. The Starbug crew later rode out the passage inside a cave system on the planet. (Last Human)
Alternatively, the crossing back to their universe went smoothly, but Red Dwarf had gone missing. (Backwards)