Hidden text and hidden links are big trouble and something you should never do.
Hidden text could be the keywords written in the same color as the background. A hidden link is simply a link which is not visible to the nacked eye, but the search engine robots would follow.
It might be a graphic which is only one pixel by one pixel and the same color as the background.
If a search engine detects text in the same color as the background, it might penalize or even ban your site. In fact, one search engine expert has even suggested that if your background is, say, white, and you have a black table with white text on your page, that search engines would read that as hidden text (white background, white text) even though the text is clearly visible in the black table.