Where you place your words on a page is just as important as the words themselves.
A web page contains several different parts — from headers, to footers, sidebars, and more. Search engine algorithms have always had the goal of finding the most important part of a page. Both Microsoft and Google hold several studies and patents suggesting content in the most relevant sections of HTML have a higher degree of importance.
For example, content found in the main body text will likely carry more weight than text placed in sidebars or any other secondary positions. At the same time, repeating text placed in boilerplate locations runs the risk of being discounted even more.
Page segmentation plays an even bigger role on mobile devices, which often hide entire parts of the page. Because search engines want to serve users the most relevant part of a page, so the content in these areas deserves the most attention.
Furthermore, HTML5 added semantic elements such as <article>, <aside>, and <nav>, which clearly define sections of your page.
Regarding on-page optimization, semantic distance refers to the relationships between different words and phrases in the text. This is different from the physical distance between words, and relates to how terms connect within sentences, paragraphs, and other elements.
For example, search engines know that the term “mustang” relates to “car type” by measuring the distance between different words and phrases within different HTML elements. As the concepts are semantically closer, so is their potential relation. Words and phrases in the same paragraph are closer semantically than phrases separated by several blocks of text.
There are HTML elements that can shorten the semantic distance between concepts. For example, list items can be considered equally distant to one another, and the title of an article may be considered to be close to every other term in document.
A good tool to semantically structure your text is Schema markup that explicitly defines relationship between terms.
Search engines also employ methods of indexing pages based on complete phrases, and also ranks pages on the relevance of those phrases. This is known as phrase-based indexing.
More importantly about this process is not how Google determines the important phrases of a page, but how Google can use these phrases to rank a webpage based on how relevant they are.
This is where search engines use the concept of co-occurrence which indicates that certain words tend to predict other words. If your main topic is around “Jamie Oliver,” this phrase will often co-occur with phrases like “cooking show,” or “cooking recipe”. Subsequently, a page that contains these related terms is more likely to be about “Jamie Oliver” than a page that doesn’t contain related terms.
Moreover, add incoming links from pages with related, co-occurring phrases and suddenly your page will receive powerful contextual signals, making it highly relevant in the eyes of search engines.