The thing about relevance in search engines is, your pages either have it or they don’t. SEO is all about the relationship of variables and relevance acts as the hub of all activity.
By the very nature of design search engines cross reference information from sentence to sentence and page to page to gain and overview of what each page in your website is about. They then compile an aggregate score to determine frequency and topical relevance from key indicators collected from various sources.
Sources such as the relationship between a title and the description and how both relate the the amount or ratio of content cross-referenced between both. The relationship between the keywords and anchor text and how many pages are internally linked within your own website and the volume of web pages linked to your pages from outside sources.
As a result, relevance is determined. This relevance is a by-product or synergy if you will, but without it, it is increasingly more difficult to overcome the opposition of millions of other pages collectively targeting the same keywords, key phrases and market segments as you.
Since laws are based on other laws, in this instance the clear advantage is that all search engine algorithms are based on the fundamental laws of language. As a result, continuity or co-occurrence is your ally.
By crafting clearly delineated segments of content (based on synonyms in language) you reinforce and fortify your relevance for the topics spanned within your website. Depending on how clear, how structured and the amount of volume and reputation each page has (within your site and referenced from other sites) rankings are produced in search engines.
The key is not to attempt to do too much with one page alone such as rank for 20 keywords). There are no penalties in search engines for creating your own content and linking to yourself within the context of your content (just consider Wikipedia as an example). The idea is to create topical pools of information (across multiple interlinked pages) which inherently based on language convey continuity to the search algorithm.
Even if your understanding of search engine optimization is minimal, the value of great content is enough for the algorithm to glean topical relevance and assign authority to your website regarding that topic.
The key is to find the benchmark within your own website and on a page to link ratio from external sources to signal search engines that your page has the appropriate synergy and relevance to rank in the top 10 results, that is the basis of SEO.
From there, once you dial in the appropriate threshold for each page through deep linking, reinforcing internal links and building a solid link reputation from other sites, virtually any phrase is possible to acquire over time.