A good quality backlink comes from a trustworthy and authoritative website. Google trusts that they do not engage in unnatural linking based on their history, and the amount of traffic and links they earn establishes a high amount of trust in the links they use. A bad backlink comes from sources that are unknown, irrelevant, and/or untrustworthy because of their own backlink profile or that they link to other websites in ways that don’t make logical sense. This is a sign that they are using linking unnaturally to try and game search algorithms for increased rankings, or to hurt a website as part of a negative SEO attack.