If you're concerned that Google's move to mobile first indexing might negatively affect your site's visibility and search engine ranking...
What are Hidden Links? Why Hide Links At All?
Hidden links and text are designed to be seen only by a Search Engine Spider (such as Googlebot – Googles version of the spider!), and as such are hidden from human users. Tactics used to include having white text on a white background, incredibly small text, links on full stops and other equally as crafty techniques!
So why so people use hidden links? Well, hidden links are just one of many black hat techniques that used to be used by website designers to help improve their rankings in the SERPs (Search Engine Results Pages). Hidden content and links would be used to deceive search engines into thinking that content was more relevant than it was, and would often contain content that would be gibbersh to a human user but had a good keyword density (using the websites chosen keywords).
As with all black hat techniques, although you may get relatively good results quite quickly they are generally short lived and will definitely get your site penalised (or removed from the Search Engines index altogether in extreme cases!) if you’re caught. As Search Engines become more and more sophisticated these techniques are used less and less, but spammers are always thinking of ingenious methods!