Since the story of Doctor Who is always moving forward and backward in time to change the fine details, its overarching narrative can be a bit hard to follow. One semi-consistent part of the story is the fate of the Doctor's home planet Gallifrey. The destruction of the Time Lords' home has been a crucial motivator for the Doctor's adventures since the beginning, but the story behind it remains interesting.
The Doctor is a strange hero in almost every conceivable way. The character who led one of the longest sci-fi franchises of all time stands out against his peers. One of the few aspects of his story that is relatively common is the fact that his home planet was destroyed. It's key to his backstory, but the details have changed a few times.
Amazingly, though Gallifrey was first mentioned in the very first serial of Doctor Who, the Doctor's home planet was not seen until the final entry of the sixth season. It wasn't mentioned by name until the 11th season, almost a decade after the serials began being released. Its first depiction bears little resemblance to the one fans are aware of. Almost nothing is revealed about the Doctor's home planet throughout the original series. Details only begin to become clear after the 2005 reboot series. In the second episode of the Doctor Who reboot, the Ninth Doctor mentions the destruction of his home planet for the first time. The name Gallifrey isn't mentioned again until the Tenth Doctor's first tenure. Since most of its canon was established in the reboot series, the story has changed a lot since then.
Gallifrey had a long and complex history, but most fans don't know anything about it because it's all depicted in the largely standalone novels. According to that canon, Gallifrey was the center of power for a massive empire that ruled over countless other planets. Its citizens were all telepathic warriors ruled over by a cultic matriarch called Pythia. Pythia evidently possessed actual magic powers of prophecy, but she was later deposed by a more egalitarian science-focused group. The best-known architect of the plan to take over Gallifrey was Rassilion, who has appeared on the show from time to time. In a rage, Pythia cursed Gallifrey so that no children could be naturally born. As a result, Rassilion and his pals created genetic looms that had the power to construct a new generation of Gallifreyans. This process also provided every fully-grown newborn with a set of 13 regenerations.
That canon, like every other, is disputed. According to the recent season 12 finale, the Time Lords were the result of genetic experiments on a native species called Shobogans. A scientist named Tecteun discovered a portal to another dimension while on a distant planet. In the other realm, Tecteun found an infant and took him home. Tecteun called this kid "The Timeless Child" because he could instantly heal from injuries and survive death by fully resetting his physiology. Sure enough, that child would later go by the Doctor. Meanwhile, a scientist called Omega, one of Rassilion's allies would eventually discover an energy source that would allow the species to travel through time. They then earned the name Time Lords. Time Lord society went on for untold ages before it all came to a heavily disputed end.
Gallifrey was believed to have come to an end in the final Time War. The Ninth Doctor was convinced that the conclusion of that conflict wiped out all the Time Lords and the Daleks. The Tenth Doctor went on to reveal that an earlier version of himself caused that violent ending. The 2013 special "The Day of the Doctor" depicted John Hurt as the War Doctor, who made the fateful decision. That special also revealed the true fate of Gallifrey. Instead of obliterating the planet, the all-powerful sentient superweapon known as The Moment actually shunted the world into a pocket dimension, rendering it out of reach. This froze Gallifrey in time, but the planet eventually found a new home. Gallifrey exists at the end of the universe, placing the planet back in play for future Doctor Who stories.
Gallifrey went from a vague background detail of the Doctor's story to a destroyed memory that gave him his motivation, to another active planet that needs saving. It's an interesting series of changes, and there's no way to tell how future stories set on the planet could go. Gallifrey exists at the end of the universe, but it's alive and well. The most recent development on the planet came when the Thirteenth Doctor saved the day once again, freeing the Time Lords from a scourge of Cybermen. Gallifrey is the original home of the Time Lords, and through a lot of new adventures, it's an important part of the story once again.