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 succeeded and he activated the device in full view of a Town Hall meeting regarding Mr Burns' recent behavior. Moments afterwards, however, Mr Burns was shot by an unidentified assailant and left in critical condition. The machine was later destroyed by the combined efforts of Homer Simpson, Lenny Leonard, Carl Carlson, Barney Gumble, Moe Szyslak, Otto, Bumblebee Man, Snake, Groundskeeper Willie and others. Tying their vehicles to the machine, they bent and then snapped it from its podium, sending the Sun Blocker tumbling down the mountain-side towards a nearby town. Screams were soon heard followed by the sounds of destruction. However, when Krusty asked what the name of this destroyed town was, Principal Skinner told them it was Shelbyville, much to everyone's euphoria (as they had a massive rivalry with the town).
According to Mr Burns, the Sun Blocker was conceived from Mankind's 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."