The backlinks are an indication of the popularity of a website. They are important for SEO as most of the search engines like Google give more value to websites that have a large number of quality backlinks, i.e., a site with more backlinks is considered more relevant than a site with fewer backlinks.
The backlinks should be relevant which means they should come from the sites that have content related to your site or pages otherwise the links if they are coming from sites that have different content, will be treated as irrelevant backlinks by the Search Engine. For example, there is a website about how to rescue orphaned dogs, and it receives a backlink from a website about dog care essentials then this would be a relevant backlink in a search engine assessment than a like from site about automobiles.
