Basically, Jean-Paul Sartre wrote the novel Nausea in order to explore some of the philosophical ideas that would make him one of the most famous philosophers of the 20th Century. More specifically, the kinds of ideas he explores in this book come from a school of thought known as existentialism.
At its most basic level, existentialism believes that all knowledge of the world is filtered through the individual human being. More specifically, existentialism believes that the universe doesn't care one way or the other whether humans are alive, and so humans are basically cursed with a need for meaning in their lives that the world can never give them. This can be bleak: just ask our buddy Henri.
You can probably see how this way of thinking is connected to Antoine's struggles to make his life seem meaningful… and his crushing depression.
But what are we supposed to do if the world is apparently meaningless? Well according to existentialism, it is our responsibility as individuals to make our lives meaningful by taking action and devoting ourselves to whatever we do, even if we know that deep down there is no meaning to it.
Like Antoine at the end of Nausea, it is our responsibility to write a book… or maybe build a bridge or paint or plant flowers or skydive. Whatever occupies your mind and helps distract you from the absurdity of life—even temporarily—is what's going to help you go on living in the long run.
Which is actually pretty uplifting, in a way. The idea that we make our own meaning is a similar philosophical statement to The Shawshank Redemption's oft-quoted line "Get busy living or get busy dying." There's meaning… but it's up to you to grab it.