It means the video won't have ads and thus the creator won't earn any money from ads. It's very difficult to make a lot of money from YouTube ads, but some people are doing it and getting "demonetized" really hurts them.
Like anything else, I think you have to find your audience. A 6 year old apparently made $11 million this year off his YouTube channel:
https://www.washingtonpost.com/news/morning-mix/wp/2017/12/1...
If you see a YouTube channel with 250,000 subscribers or more, they are probably taking in around $10,000 a month from YouTube (assuming they post regularly.)