I think "Body content relevance" is the written "non-image" text on the page of the web site which is descriptive in nature and relates to the title, description and keywords. It is not mandatory to have relevant body content, but it most definetly will assist your ranking on the search engines and yours thinks.
body content relevance is a kind of feature of content according to which all the text and pictorial information provided should be related to our tiltle and keywords we r using.