From what I understand, the core of it is that a certain amount of time ago (before recorded history), an alien (named Xenu, I think) came to Earth and dumped a bunch of aliens that he’d gathered all around the universe into Earth’s volcanoes and killed them with nuclear weapons. The ghosts of these aliens became trapped on Earth as “ingrams” or some such nonsense, and that the “ingrams” are responsible for any bad things that happen to us. The goal of scientology is to purge your existence of ingrams by giving the church or scientology vast amounts of money. The “church” then hooks you up to a machine that checks you for ingrams, and charges you more money to teach you how to purge yourself. After you spend some time purging yourself, you pay more money to get checked again, and so on and so forth, until you’ve purged enough ingrams to become an “operating Thetan”. You then pay more money to get into the higher levels of the church, where you are rewarded with the right to pay more money to do super secret Thetan stuff. It was created by science fiction writer and pathological liar/con man L. Ron Hubbard, whom scientologists claim is still alive.
I have a brother-in-law who explored scientology in the Phoenix area in the early 2000s. He’s now literally insane and institutionalized.