Thematic Relevance is a tricky concept, but hopefully this example will help you follow along. When your pages are tied together with specific, targeted keywords, it establishes a theme throughout the site that makes your site as a whole appear more relevant to those search terms.
For instance, if you’re writing about Christian Louboutin Black Patent Leather Pumps for your shoe store online, your main focus for the content of that page will be the keywords associated with that specific product. That being said, you also have the opportunity to use that content as a means to strengthen other pages on your site, specifically those that exist at a hierarchically higher level to that product. The pumps in question may be part of your Christian Louboutin Shoes category, for which you have a category page optimized for the particular term “patent leather shoes.” If you use the term “patent leather shoes” on your product page, then you’re actually helping your category page rank for its term as well.
The way Google see it, the entire category or section of pages share a common theme, and the main theme is represented by this category page. At the same time, if your homepage is trying to rank for the term “shoes”, your use of this term on your product pages will help as well. This makes the term “shoes” the most thematic keyword on the site.
Still confused? Here’s an example.
A visitor lands on your home page. The keyword on the home page is shoes, so your content should include that keyword (preferably used with an optimal keyword density, and preferably used within at least 200 words).
This visitor (who clearly has excellent taste) clicks on the Christian Louboutin category page. This category page is targeting the keyword phrase, “patent leather pumps.” The content on this page must include that keyword. However, the content should also include the word shoes a few times – not as a keyword, like “patent leather pumps,” but just as a part of the text.
Next, the visitor explores the Christian Louboutin page with glee, and clicks on the iconic black pumps. The copy on the product page is targeting the keyword phrase “designer black pumps,” but as with the category page, it would be wise to mention the keyword phrases from the hierarchically higher level pages in the copy (shoes and patent leather shoes).
Don’t let this principle scare you. Most sites actually do this without realizing it, because the site centers around a theme anyway (what business / product you’re selling or the information you’re providing). However, it certainly helps to be mindful of Thematic Relevance when creating copy.