The basic idea of NaNoWriMo is that you write about 1667 words every single day for the entire month of November. If you do exactly that, you'll complete a short novel of 50,010 words by the end of the month.
"What I Know---There's a secret that real writers know that wannabe writers don't, and the secret is this: It's not the writing part that's hard. What's hard is sitting down to write. What keeps us from sitting down to write is Resistance." The War of Art; Pressfield 2002
That's all just a fancy way to say you get together in a room with other writers, you write, and because you don't want to be the only one not writing, you write more. And what happens when you write? Yup, you guessed it--words get on the page and magic will begin to spill out.