A Stasis Leak was a temporal anomaly that allowed travel in time, caused by a malfunctioning stasis field.
When Dave Lister found a photograph of himself marrying Kristine Kochanski in the past, he realised that there must be a way to travel back in time. He remembered his bunkmate Arnold Rimmer had once seen his own head telling him that he had come from the future, that Rimmer had dismissed as a Titan Mushrooms-induced hallucination, so Lister read Rimmer's diary about the incident. The head had stated that there was a Stasis Leak on Floor 16, but Holly could not explain what that was. Lister, Rimmer, the Cat and Holly traveled down 2,567 floors in the Xpress Lifts to Floor 16 to see what might be there.
They found a glowing hole in the wall next to the unused stasis pod (not the one Lister was in for three million years) through which they could step. On the other side, they found themselves in a shower room on Red Dwarf, materialising approximately three weeks before the radiation leak which killed the crew. The Cat demanded an explanation of what the Stasis Leak was. Rimmer suggested that it was a rent in the spacetime continuum or a singularity where normal laws did not apply. Lister speculated that when a stasis field escaped the booth that it would preserve whatever it had leaked into, like the shower room. The Cat did not understand any of these explanations, until Lister said it was simply "a hole into the past", which The Cat took to mean "a magic door". Embarrassingly, there was a man in the shower.
They soon determined that they could travel back and forth at will, but anything they tried to bring back to the future (besides themselves and whatever they had on them when they went back in time) would be turned to powder. Lister realised that they would be able to save one person by convincing them to use the spare stasis booth. He tried to ensure that Kochanski survived, but discovered that a version of himself had travelled back from even further into the future to marry her. Rimmer tried to convince his past self to go into stasis, but he was unsuccessful because the past Rimmer thought he was hallucinating from ingesting Titan Mushrooms.
When Lister had his kidneys stolen by a deranged droid, the Dwarfers initiate a complicated plan to find him some more organs by traveling into the past. Luckily, a snack dispensing droid named Snacky - who they had rescued from Asclepius' Station - had once overheard a detailed explanation on how to hack a stasis field to create a time portal by listening to the late scientist Romero Gonzalez.
This method was similar to creating a stasis leak, although it was deliberate instead of accidental, and very difficult to accomplish. Nevertheless, Snacky surprised everybody by hacking an old stasis booth on B Deck of Red Dwarf and successfully created the time portal for the Dwarfers to pass through. (Give & Take, Series XI)