Hidden text is a popular blackhat SEO technique where text is hidden from users. The page is usually stuffed with tons of keywords that are thought to help the ranking as the page now has a higher keyword density and more copy about the theme of that page.
This used to be accomplished by making the text the same color as the background or by making the text super small. It is becoming more common now for sites to use CSS to hide certain elements of the page completely from its users. Since search engines don't crawl CSS that is included on a page (obviously not a confirmed fact) they end up getting all the extra keyword text like they would with any other document. And when a user browses the page they notice no effects at all as the spammy text is not displayed for them.