The Stargate franchise is a series of films and TV shows which are based in the Stargate universe. The first film, Stargate, was the one that started the franchise. It starred Kurt Russell as a military commander who is sent through the eponymous Stargate to visit another planet.
The idea is that the Stargate was found during an archaeological excavation of ancient Egyptian ruins. The Stargate was an advanced piece of ancient alien technology which worked as a device which would send people or objects to another planet. On the other side of the Stargate was another Stargate which was located on an alien planet.
The back story is that the pyramids were built by ancient humans who were enslaved by the alien race using the Stargate to travel back and forth between their planet and ours. There was a revolt where the humans turned against their alien overlords and buried the Stargate. The Stargate then waited for thousands of years to be discovered by the archaeologists.
The whole idea is based around a theory called the “ancient astronaut” theory, which was popularized in the 60s by Erich von Däniken in a book called “Chariots of the Gods.” In this book Erich hypothesized that ancient alien beings had traveled to the Earth. Then they taught ancient man how to build pyramids and other hallmarks of civilization.
In the film an Egyptology professor, played by James Spader, works together with Kurt Russell’s military commander character to travel to the alien planet. Once they arrive they realize that they cannot return to Earth. This is because the Egyptology professor, who deciphered the hieroglyphics which control the Stargate, is unable to figure out how to send everyone back to Earth.
After the Stargate film came out in 1994 it was rather well received. Then, in 1997, a Canadian production of a TV series based on the Stargate film was created. The TV series was called “Stargate SG-1” and was based around the same characters from the film. In 2004 another TV series called “Stargate Atlantis” was made. In 2009 another TV series in the franchise was made, called “Stargate Universe”
The idea for each of the different TV series was that there were many Stargates that they could travel through. Therefore the characters would spend each episode visiting a different planet through the Stargate. The Egyptology professor character would align the Stargate using the hieroglyphics in order to tell the Stargate on Earth to send them to a different planet each time.
The premise is that the alien race who had enslaved the humans in ancient Egypt were an interstellar species. Therefore they had traveled all over the galaxy and used the Stargates to travel between different planets. The Stargate is a useful device as the speed of light, which limits all travel in the universe, means that it can take hundreds of years to travel between different stars.
By including a device, like the Stargate, which allows instant travel between different planets, the creators of the franchise have allowed for a way for the characters to visit different planets. This is without the hundreds of years it would take in real life. In real life the characters would have died of old age before they arrived at the different planets.