42 is a quotation from the science-fiction novel Hitchhikers Guide to the Galaxy by the famous Douglas Adams.
In his book the number 42 is the answer to the meaning of life, the universe, and everything - calculated by a very big computer over a very very long time:
All right, said Deep Thought. The Answer to the Great Question
Of Life, the Universe and Everything said Deep Thought.
Is said Deep Thought, and paused.
Forty-two, said Deep Thought, with infinite majesty and calm. Douglas Adams The Hitchhikers Guide to the Galaxy quoted from GoodReads
Software developers like to use 42 as magic number, thats a number without any real significance, which you could replace by any other number. Of course the name arc42 wouldnt sound half as cool if we had choosen another number