Hidden Content is a black hat SEO technique used to add content to your site which will not be visible to your visitors but search engines will read it. This content can be used to "set" more keywords to you page, or adding a hidden paragraph which will help be ranked on search engines results.
If this technique detect on your site, it will cause you big troubles and it might be banned too.
This can be done with almost every programming language and some webmasters are taking the risk to use it.
For example, if you are writing a script you can use <noscript> tag to write your keywords, i.e. <noscript> SEO, search engine optimization, back links, high ranking, building back links, SEO Services, Hire a SEO expert </noscript>
Comment tags are also used for this purpose i.e. <!– How to earn a lot of money in a few hours using your internet connection. ->