The Sun Blocker was a device developed by Charles Montgomery Burns. As the name suggests, it was created to literally blot out the sun outside, in order to ensure Mr. Burns' electricity was sold in a monopoly, covering Springfield in perpetual darkness otherwise.
The machine was extremely infamous alongside the theft of Springfield Elementary School's oil well for riling the Springfieldans' hatred of Mr. Burns, enough to want to murder him, especially Smithers, who had earlier been fired by Mr. Burns when the former protested against the plot and cited it was too heinous of a plan for even him to nominally support.
Ultimately, Mr. Burns' plan had succeeded and activated the device in full view of a Town Hall meeting regarding Mr. Burns' recent behavior. Moments afterward, however, Mr. Burns was shot by an unidentified assailant and left in critical condition. The machine was later destroyed by Homer Simpson, Lenny Leonard, Carl Carlson, and Barney Gumble, where they snapped it in two and sent its remains down into Shelbyville, totaling the city to their delight.
According to Mr. Burns, the Sun Blocker was conceived due to Mankind overall having an innate desire to destroy the sun, and that, due to the infeasibility of actually doing such an act, he settled for "the next best option."