Here's the breakdown:

* The Vogons are a race of bureaucratic, emotionless aliens who work for the Galactic Hyperspace Planning Department.

* The Vogons are building a hyperspace bypass. This bypass needs to go through the space where Earth is located.

* Earth is in the way. The Vogons don't care about the planet's inhabitants, they only care about completing the bypass.

* The Vogons destroy Earth with a "small, insignificant" planet-destroying device, without any warning or explanation.

The reason for the bypass is never fully explained, but the Vogons simply state it's "necessary" and "important" for their work. This highlights the absurd and bureaucratic nature of the universe Adams portrays.

This event is a pivotal moment in the story, as it kicks off the adventures of Arthur Dent, a hapless Englishman who is one of the few survivors of Earth's destruction.