An external link is any backlink or hyperlink that redirects the user to another domain from the domain it was initially clicked in. In search engine optimization (SEO), this term is used for any link that redirects from any website to another; this can be accomplished through payment or not.
Adding an external link to a website is easy as copying it and pasting it where the writer wants it to be, but there many ways why it should be considered a basic Search Engine Optimization (SEO) practice, the most important could be giving credits to the sources the writer used to make his content.
The quantity and quality of those external links matter; it is recommendable to add links from authoritative sites that are trustworthy, informative, and relative to the content of the site, links that are high quality will improve the credibility of a website and will make most likely to appear in search results and rank higher, but if otherwise, the links added to a website are low quality, spam or invasive links, this will hurt a site’s reputation and will probably rank lower in search results.
External links have more weight to search engines if they are to relevant pages that are somehow related to the site that is placing them. Adding links from authoritative pages will make the site improve its authority.
External links are among the most important metrics for search engines to position a website in a high position. This is probably because linking has always been a part of the web and is a reliable and trustworthy method to measure a site's content and determine if its information could be important for the users.
It is very important to note that search engines value quality over quantity, but this does not affect the fact that external links remain an important measurement for ranking sites in the search results. It increases the possibilities of a search engine accessing a website to qualify it.
Adding external links to a site is really effective on its own to make a site rank high in search engines, but there are some things to consider increasing its effectiveness even more:
The more authority a site has, the more valuable will be a link to it; the search engines not only navigate a website but also the links it contains at the sites they redirect to.
See also Domain Authority
The relevancy of the site a link redirects to is a factor to note. The links placed on a website should have something to do with the content presented in it.
The quality always comes before quantity, but it is true that if two sites have both high quality and relevant links, the one with higher volume will most likely rank higher.