Take all the keywords your competitor ranks for and subtract the keywords that your own website ranks for. What you get is a list of keywords that you should be targeting.
Ahrefs’ Content Gap tool allows to do that with a bunch of competitors at the same time, giving you a massive list of keywords that you should be targeting:
You have a few options for filtering results:
The keywords that any of your competitors rank for
The keywords that at least x of your competitors rank for
The keywords that all your competitors rank for
You’ll get the best results if you choose to see the keywords that all your competitors rank for in search results, but you do not:
Content gap analysis is one of the quickest ways to find hundreds of great keywords that your website should be ranking for.