At its most basic, a null result is a result without the expected content: that is, the proposed result is absent. Unsurprisingly, visitors who receive such outcomes after a search are more likely to leave your site and, more often than not, will never return. As a rule of thumb, if you are experiencing anything above a 5% null-result rate, there is room for improvement.
But, by leveraging your null-result keyword list and either reducing or eliminating all keywords present, you can reduce bounce-rate, increase conversions, and ensure satisfied visitors return time-after-time.
The most common way to identify your null-result keyword list, and leverage it to your advantage, is to send an event to your analytics tool, such as Google Analytics.