Backlinks are important for SEO because they signal to Google that another resource finds your content valuable enough to link to it within their own content. As a website earns additional backlinks, search engines infer that the website possesses valuable content worth ranking well on the SERPs
Backlinks occur when one website links to another website. Whenever backlinks occur (particularly quality backlinks), it is like receiving "a vote" for a webpage. The more votes you get from sites with authority, the more positive an effect on a site's SEO (rankings and search visibili