You can see the HTML source code for a hidden image link in the screenshot above, basically the black hat SEO had added an image link with an image file that was tiny and see through (transparent).
No one viewing the webpage would see the hidden link, this is against the Google webmaster guidelines.
This is a very old black hat SEO technique, Google will index the link and count it as a backlink, but if caught expect a Google penalty.
Continue Reading Black Hat SEO Hidden Links