You do not need to have read the book. Here is the whole story in three steps.
The Hitchhiker’s Guide to the GalaxyEntry: Earth
Harmless.Mostly harmless.
Harmless is not the same as unimportant
Earth looked like nothing and held the keys. Unintimidating things do that, given time. A well-made message can too.
Big does not have to mean harmful
Hence the whallephant on our logo: part whale, part elephant. Two of the largest animals alive, and mostly harmless. Doing big things should not cost the planet.
Nobody gets to zero
Being alive means using things up. The aim is not purity; it is that everyone tries to be at least mostly harmless. Enough people doing that and the world gets better. We start with the work we take on.
So: MostlyHarmless. A joke from a good book that turned into a standard. The whallephant is there so we do not forget.