What is Keyword stuffing?
It is considered to be a black hat seo technique. Keyword stuffing occurs when a web page is loaded with keywords in meta tags or in content . Due to this activity, the importance of meta-tags is lost completely.
It is used to unethically to reach the top rank in the search engines and visibility for particular keywords. A word that is repeated too often may raise a red flag to search engines.
Hiding text out of view of the visitor is done in many different ways
1) Text colored to blend with the background, CSS “Z” positioning to place text “behind” an image – and therefore out of view of the visitor – and CSS absolute positioning to have the text positioned several very far from the page center.
2)”Noscript” tags are another way to place hidden content within a page. While they are a valid optimization method for displaying an alternative representation of scripted content, they may be abused, since search engines may index content that is invisible to most visitors.
3) Inserted text sometimes includes words that are frequently searched (such as “sex”), even if those terms bear little connection to the content of a page, in order to attract traffic to advert-driven pages.

