Hidden text is any text on a webpage which is crawled by the search engines but is not visible to the user. The use of hidden text is usually a black hat SEO tactic.
Hidden text was often used by spammers to stuff a page with keywords while not distracting the user. Two of the most common techniques were typing a keyword over and over again in the smallest font size available then covering it with an image.
Another common method was to hide keywords by making the text the same colour as the background.
The SEOniverse would advise you not to use hidden text as a tactic to rank your webpages – it simply doesn't work anymore.