relevance

extent to which the content of a website page is related to a search index query

relevance

Your website may be more relevant (to a given query of the search index) than another, similar website, if it answers a question that somebody asked a search engine whereas the other website doesn't answer the question.

Search engines like Google exist to help people find answers to questions amidst billions of web pages, each a possible answer (most of them irrelevant or wrong answers). The task of search engine optimization of a website is to help the index understand the answers you offer (content, products, services) and match your pages to the questions people ask when they are looking for something like what you offer.