Hello Ana! Nanowrimo is an acronym for National November Writing Month. It's a website: www.nanowrimo.org It's a challenge where you try and write a whole novel in one month: November.
Before the challenge begins, you set your word count goal. (for example, you might want to write a 50,000 word novel in the month, or a 15,000 word novel, etc.) There is a place where you can update your word count whenever you write more of your novel. To win NaNoWriMo, you achieve your word count goal and finish your novel. On the website they have a forum similar to this one, to talk about your writing, and other related stuff. It's basically done for fun and for a challenge.
I hope that helps!
I was sure it was November! Goodness, I have been less than helpful here! 