If you have never heard of SEO, it's an acronym for Search Engine Optimization and according to Wikipedia.
Search engine optimization (SEO) is a set of methods aimed at improving the ranking of a website in search engine listings, and could be considered a subset of search engine marketing. The term SEO also refers to “search engine optimizers,” an industry of consultants who carry out optimization projects on behalf of clients' sites. Some commentators, and even some SEOs, break down methods used by practitioners into categories such as “white hat SEO” (methods generally approved by search engines, such as building content and improving site quality)
Hidden links are a great way to boost your rank on a specific keyword. They are usually in a paragraph, then the keyword would have a link to the page you're looking to boost it's rank, but without an underline and the same color as the text.
As an example, I'll take SEO. I would write a paragraph in my site about SEO and a link to my SEO page, though when you'll read the text, you won't notice SEO as a link because it's the exact same color. The search engines consider that no problem as it's a link with specific keywords to a page and we all know search engines love it!
If you have any questions about this — Just post a comment and we'll help you!