In the book there are a lot of meaningful things and lot's of meaningless things the main meaningful thing is the planet called Magrathea which I will talk about in the first paragraph. Meaningless is a tough one because it is hard to choose which things are actuly meaningless but int is is something that has no meaning which is pretty self explanatory.
Meaningful
Magrathea is a sign of a very Meaningful thing because in the book Magrathea is a planet that makes custom planets that you can make whatever you want on them, Arthur and Ford have been searching for Magrathea so that hey can make a whole new earth to live on but they have to find it first. The Magratheans are people who value money very much that is why they make and sell custom planets.
Meaningless
In the book Douglas Adams has a lot of meaningless things going through the book because in some parts especially the part in chapter 18 when the whale just randomly appears in the sky and starts falling to i'ts death that was just really meaningless and made no sense whatsoever to me and probably not to anyone else ether.
Just to recap over that, Douglas Adams has a strange way of writing and it involves meaningful & meaningless things that sometimes make the book just a little bit more interesting, but sometimes the book takes a change and it moves to other things like just some random and slightly meaningless but very absurd things. The whale could be put under both of those categories.