A requested external link is a link you ask someone to place on their web site to link to your web site.
A requested external link is a link you ask someone to place on their web site to link to your web site. These external links are often part of a link exchange and may be referred to as reciprocating links (usually if a requirement for the link is that the other party have a link back).
Requested external links can be beneficial to the SEO (search engine optimization) of your site. However, these usually are not as beneficial to your site's SEO as unsolicited external links, in part because these links are usually grouped together on a page with a long list of links, and generally do not have meaningful context that is a major part of the value of unsolicited external links to the search engines. But there is value and benefit from these links in improving the "ranking" of your site in the search engines, just generally are not quite as good as the unsolicited links.
A link exchange, where you link to their site and they link back to yours is actually something that the search engines look for, and while these links are still of value, the external link that is part of a reciprocating link exchange is not as valuable in improving your ranking as it would be if it were a one-way link.