The relevance of your site and it’s content based on keywords.
When I search for “diapers” I won’t get any results for FedEx. That’s because FedEx isn’t trying to rank for the keyword “diapers”.
However, if I search for “shipping”, FedEx will show on the first page of results. Why? Because FedEx is trying to rank for the keyword “shipping”.
The more relevant your content is as it relates to words and phrases people search for, the liklier your site will be shown in the results - driving more and more traffic.