A striking distance keyword is a search your site ranks for on page two, roughly position 11 to 20, close enough that a better page can reach page one.
A striking distance keyword is a search your site already ranks for on page two of Google, roughly position 11 to 20. The page is close to where the clicks are, and Google has already judged it relevant, so a better page can often move it up.
The reason it matters is how clicks are spread. In Backlinko's study of 4 million results, only 0.63% of searchers clicked anything on page two.
You find them in Search Console's Performance report by filtering to an average position between 11 and 20. The full method is in how to find searches you almost rank for.