This was when internet trolls took on a cult with the internet; it's as boring as it sounds exciting.
Scientology was a religion that was founded in the 1950's. It gained controversy for censoring it opponents, banning many facets of medicine, mistreating people who within the church and apostates, and just being generally crazy. Anonymous was a group of Internet Hackers who tried to do things to help the world, usually involving hacking.
Of course, Anonymous decided to go after Scientology. This was Project Chanology, based on a portmanteau between chan (of 4chan, where Anonymous formed) and ology (of Scientology). This lead to the reveal of Operation Snow White, a Scientology intrusion of the government to remove documents that made the religion look bad.