Hello,
     Can anyone know about "Hidden Text" of seo...which is useful to seo or not..?

Hidden Text is not useful in SEO. It is part of black hat SEO. It mean same background and same text.

"Hidden thing of Seo" is used to load a Web page with keywords and keyword phrases. Not seen to visitors but this would improve the page's rankings in the search engine results. Hidden Things of SEO is identified as search spam by each of the major search engines. So avoid it.

Hidden text means misguiding the crawler. Its a kind a black hat seo technique. It's Highly Not recommended these days or your site would be penalized by google.

Hidden texts are consider as a black hat Seo techinique. Your site may get penalize for that.
try to abide with google webmaster guidelines

Hidden text is textual content which your visitors cannot see, but which is still readable by the search engines. The idea is to load a Web page with keywords and keyword phrases that would be unsightly to visitors but that would improve the page's rankings in the search engine results, and to do so without letting your visitors see the text. Hidden text is identified as search spam by each of the major search engines.

Hidden text is not seo friendly for search engines. Better stop it, once google come to know about hidden text surely it may spam the site.

Hidden Text is a spam or black hat technique it is bad for a website & search engine.