In order to use this technique you have to have site search on your site and added the parameter in your view settings.
You do that by going into Admin -> choose a view -> Click on View -> write the in the search parameter, for wordpress the default is s.
After tracking your site search for a while you can identify issues on your site by going into Reporting -> Behavior -> Search Terms.
Then choose secondary dimension and search for “Refined Keyword”.
What this shows you is the second search people perform after they searched once, why it is extra interesting is because the user clearly think the information should exist on the website they just need to find the right keyword to search it!
This a great way to spot issues in the UI and also spot content possibilities. If a lot of users search for a specific item on your site, you might consider to write about it or start selling it if you are running an ecommerce site.