What is link equity?
Link equity is often referred to as link juice, and it represents the influence which that link passes from one page to another. Before the introduction of "nofollow" links, each link carried with itself some value, so the practice of spamming (linking as much as possible, not with the intent of informing but to gather traffic) was widely adopted.
During that period, it was not unusual to see several websites owned by the same person at the top of the search rankings, thanks to mostly useless and irrelevant links that lead to content without any connection to the theme of the website. Now it is possible to navigate using links with the "nofollow" attribute, which search engines ignore when indexing internet content, and that way they neutralize any value those links might carry with them.
The amount of value a certain link is carrying with itself is determined by using several criteria. Links coming from relevant content and respectful websites carry with them greater value than the links with irrelevant content, links coming from websites with a shady reputation, as well as links from headers and footers of pages.