Backlinks are links from other sites or pages sent to a page or to the main domain. Maybe this explanation is not too good, the backlinks support a site for search engine searches.
Normally, the main intention is not to search, but to refer to a site or a page of a site. The more a website or a page has more backlinks, the more the authority and relevance of a site or web page increases compared to other pages or similar sites.
Backlinks can be created naturally, organically ... that is, links to your own site or page are given due to the relevance of the content or because it refers to relevant content.
Another way to create backlinks is the intentional one, the will. Directory subscriptions, link exchange, buying links from reviews, social and collaborative communities are ways to build links to a site.