What does “Principals Only” mean in a job posting?

Sometimes I see ‘Principals Only” in online job postings. I don’t know if I can apply to these jobs or not. What’s this all about?

What's a Not For Profit?

I feel pretty dumb asking this, but what exactly is a “not for profit?” I keep hearing I should look for a job at a not for profit, but I don’t even know what it is. And are there paying jobs or is it all volunteering?