what is back link in seo optimization?

[Anthony]

Backlinks, also known as incoming links, inbound links, inlinks, and inward links, are incoming links to a website or webpage. In basic link terminology, a backlink is any link received by a web node (webpage, directory, website, or top level domain) from another web node.

Inbound links were originally important (prior to the emergence of search engines) as a primary means of web navigation; today, their significance lies in search engine optimization (SEO). The number of backlinks is one indication of the popularity or importance of that website or page (for example, this is used by Google to determine the PageRank of a webpage). Outside of SEO, the backlinks of a webpage may be of significant personal, cultural or semantic interest: they indicate who is paying attention to that page