Visible text content
"Content is King" is a mantra often repeated with regards to search engine rankings.
This would seem to be obvious, given that the search engines' primary function is to provide relevant results - thus a search for the phrase
museum
should bring up a set of SERPs featuring web sites about museum-related issues.
Whilst it isn't entirely the case that "content is King" anymore , it is still a major factor and is a good place to start in terms of ensuring your site is relevant to your chosen phrases (see the Keyword Research section).
The term "visible" within the phrase "visible text content" refers to the fact that the search engines want to be able to be sure that a site visitor sees the same text as their spiders do.
Thus the "visible" nature of the text is simply that it is what you or I see on the page when we browse the site. It does not include the background code that determines layout, positioning etc.