what is keyword stuffing and hidden links? I want full explanation.

Keyword stuffing is using keywords more times in meta tags or content and hidden text is you can hide the text in the content, these two are black hat techniques for SEO, use the white hat techniques like use 4-5% of keywords in content.

The practice of loading a webpage with keywords in an attempt to manipulate a site's ranking in Google's search results.

Keyword stuffing is a deceptive technique to try and elevate a website's search engine result ranking by including targeted keywords more often than they naturally would appear in a quality document. The intent is to make it so that it's it is unlikely to be seen by a visitor but will be visible to search engines.

Keyword stuffing refers using keyword again an again where as in hidden content keyword are used many times but it does not see by visitors but crawler can read it.Both are these techniques consider as black hat seo and can leads penalization of site.

Keyword Stuffing:
When you use your targeted keywords again and again in limited space of your web page like Title, Meta Description.. etc.
For Example
You have limit 65-70 characters for Title and when you use your main keyword again and again in this limit, its called Keyword Stuffing.
<title>Keyword1,Keyword1,Keyword1,Keyword1..</title>

Hidden Text:
When you use the Font color of your web content as same as your back ground color, then your text is not visible and it is called Hidden Text.

Google Count both techniques in Black Hat SEO and not allow to use it. If your website caught to use it, your website may ban or penalize, so try to avoid it.

HI
when keyword is repeated again and again in the content its called the  keyword stuffing, the idle keyword density is 3-4 keywords per 100 words, and the hidden text is that, when the color or the text is same as the background color, these both are the black hat technique so its better to avoid them.

hi.
Keyword density must be 2-4% and in case if density is more than this percent then it will call as keyword stuffing or it may also considered as a black hat seo. HIdden text means to hide text in a content. Both are type of black hat seo. Always prefer white hat seo.

Keyword stuffing is a signpost of a very low quality spam site. Keyword stuffing is something Google clearly recommends you avoid otherwise your site might get kicked out of Google completely.

Keyword stuffing is the overuse of keywords in a piece of content. Generally, this is repeating the same keywords over and over just to achieve a higher search engine ranking.

Keyword stuffing is a practice in which webmasters load their pages with instances of commonly searched for keywords.

Keyword stuffing is a deceptive technique to try and elevate a website's search engine result ranking by including targeted keywords more often than they naturally would appear in a quality document. The intent is to make it so that it's it is unlikely to be seen by a visitor but will be visible to search engines.