NaBloPoMo stands for "National Blog Posting Month." The goal of NaBloPoMo is for participants to write a post a day for an entire month.
Although November is the biggest month for NaBloPoMo (with prizes and everything!) people are invited to take up the challenge of writing a post a day during any month.
To help inspire bloggers, NaBloPoMo assigns a theme for each month, and also offers daily writing prompts, but these are optional. Bloggers are not required to write on a specific month's theme or a specific day's writing prompt. Any post about anything at all counts.
For additional information including the rules, and how and where to sign up, please see the NaBloPoMo FAQs (Frequently Asked Questions.)