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
2 CSS has been used to make tiny hyperlinks
3 The link is hidden in a small character - for example, a hyphen in the middle of a paragraph.
The links which human cannot see but spider/bot can. It is an unethical technique which create problem for your site. It is one type Black hat SEO technique. It is harmful for your site.
Hidden links are an effective SEO technique but eventually Google will penalise some/all of the the sites involved.
When you put hyperlinks in your site whic are not visible to the visitors in order to increase popularity is called hidden links. The hidden links which will be shown as highlighted text can help to rank webpage higher for the matching that phrase.
Hidden links means those links which are crated for the search engines not for the readers , these links are invisible to the reader and they just designed for the ranking . This is black hat seo which will penalize your site .