For pretty much all of literary history, if you as an author have opted to self-publish, that's been a huge stigma. It said you weren't good enough to get a publisher, it said you very likely haven't had a professional editing it, it said you basically don't know what you're doing. You don't get into bookstores, you don't get in front of book reviewers, you don't have an advertising budget, your book gets doomed to obscurity as a result, and pretty much you suck to the point where you would have trouble finding a publisher in the future just for the fact that they knew you had to resort to self-publishing.
Then came e-books.
Now, I'm not an e-book person myself. Prefer the hard copy. Personal preference. It seems like the kind of thing I'd use maybe three times and leave in a drawer. But there are a lot of people that do use them. E-books, by definition, are available online. And the funny thing about things online is that every so often something goes viral. Little advertising is needed, as word of mouth spreads about whatever's just become wildly popular.
Which means, if you have to self-publish, you now have a semi-viable route to actual success: make your book an e-book, spread the word online, and hope you catch a tailwind. And should you manage to make it work, there's one big upside: no publisher taking a cut of the profits. You get all the money. Or if you want, the publishers might come calling later on, after they see the book's already a success.
The best route is still by far to get a publisher. But it's not totally hopeless anymore if you can't get one.
Alexandra Alter of the Wall Street Journal here profiles one such self-publisher, Darcie Chan, and her book, The Mill River Recluse.