The title of the book is also the first thing a person sees upon logging into OASIS. James Halliday, the creator of the OASIS, was inspired by the video games of his youth, which greeted the player with this phrase. The author, Ernest Cline, got the phrase from the CAPCOM game Black Tiger, which features in the novel (source).
It's not just a clever allusion, though. It also means something to the reader, whether you're looking at the book at the bookstore, the Kindle store, or on your coffee table at home. It's almost daring you: are you ready for a wild adventure that will keep you up all night and on the edge of your seat, just like the classic video games that inspired it? If you are, then come and get it.