Links were the first major factor in the “off-page” ranking (External SEO optimization) used by search engines. Google was not the first search engine that considered links to be “votes,” but it was the first search engine that heavily used link analysis as a way to increase relevancy.
Despite the chatter around other signals, links along with content remain the most important external signal for Google search rankings. But, as you will find, some links are more valuable than others.
Call quality
If you were sick, what would you trust more, the advice of five doctors or 50 random people who offered their advice as you walked down the street?
If you did not have a really bad experience with doctors, you would probably trust the advice of doctors. Even if you get fewer tips, they will come from experts. The quality of their opinions is better, therefore they have more weight.
This works the same with search engines. They will count all links pointing to websites (except for those blocked by nofollow or other methods), but they do not count them all the same way. They give more weight to links that are considered better.
What is a quality link? You will recognize them when you see them. A link from any major reputable site will be higher on a quality scale than a link that you can get by commenting on a blog. In addition, links from sites that are directly related to your site may also be of greater importance.