What YC earnestly wants you to believe is that if you just have a really good idea and nothing else, you should apply because they might be able to help you get started.

What it means in practice is that they'll receive probably tens of thousands of applications but only be able to accept 20 (https://news.ycombinator.com/item?id=9917491), so your competition will be fierce.

(BTW, see also this note from sama: "We will be slightly more open to solo founders because we think at this stage you still have a good chance of finding a cofounder. But the bar will still remain higher for solo founders given all of our historical data.: -- https://news.ycombinator.com/item?id=9917470)