If your keyword is not related to your ad and the content of the page to which you're trying to send your visitors, you may see that message.
Also, if the keyword is too general, even if it's related to the ad and the landing page, it may also be flagged as poor, because relevant for many things actually means relevant for none of them.
One recent example was the keyword 'paint' for a website selling wall paints. If you google "paint definition", you'll see the many meanings it may have. This is a classic example of a keyword which may be relevant for a human, but with multiple meanings for Google. In such cases, more detail is necessary. The former poster cited "burnt sienna paint". Now that's a lot closer to the target. That cannot mean make-up, and also cannot mean the piebald horse.