An inbound link (or backlink) is a hyperlink that points from an external, third-party website to a document or page on your website. It is a fundamental element of the architecture of the World Wide Web and is the basis for how search engines have historically assessed the authority of websites.
Conceptually, an inbound link functions as a “vote of trust” or “editorial recommendation”. It is a signal of authority – the more authoritative the source linking to you, the greater the value it conveys (the so-called Link Juice or PageRank). Accumulating a quality and natural link profile (consisting of inbound links) is at the heart of off-site SEO.
Search engine algorithms analyze not only the quantity, but also the quality, relevance and context of inbound links. Links from thematically similar, high-quality and trusted domains are valued much more than a large number of links from low-quality sources.