Hidden Texts and Links - Whats so bad about it?

To easily understand why a site can be removed from the Google indexing i.e. will not appear in Google SERP (Search Engine Results Pages) is to discern the meaning of the word "hidden" defined and classified by Google Search Team. In other words, its publishing smells "fishy" which was meant for search engine indexing ONLY,  instead of for reader's viewing.

OK, let us see, read, and observe what Google says about it?
Hiding text or links in your content can cause your site to be perceived as untrustworthy since it presents information to search engines differently than to visitors. Text (such as excessive keywords) can be hidden in several ways, including:
  1. Using white text on a white background
  2. Including text behind an image
  3. Using CSS to hide text
  4. Setting the font size to 0
Hidden links are links that are intended to be crawled by Googlebot, but are unreadable to humans because:
  1. The link consists of hidden text (for example, the text color and background color are identical).
  2. CSS has been used to make tiny hyperlinks, as little as one pixel high.
  3. The link is hidden in a small character - for example, a hyphen in the middle of a paragraph.
Source Link: Webmaster Tools › Help articles › My site and Google › Creating Google-friendly sites › General guidelines › Hidden text and links 

Understanding the above explanations, thus, the term for "Keyword Stuffing" is a similar thing to it.
"Keyword stuffing" refers to the practice of loading a webpage with keywords in an attempt to manipulate a site's ranking in Google's search results. Filling pages with keywords results in a negative user experience, and can harm your site's ranking. Focus on creating useful, information-rich content that uses keywords appropriately and in context.
To fix this problem, review your site for misused keywords. Typically, these will be lists or paragraphs of keywords, often randomly repeated. Check carefully, because keywords can often be in the form of hidden text, or they can be hidden in title tags or alt attributes.