Link insertion is the practice of adding your website's relevant link to the existing content of another website. The link insertion backlink strategy typically includes outreach to authoritative and topically relevant websites in an effort to improve the reader's experience. The result of link insertion is a backlink from another page to your page that provides additional content and information to the reader of the parent topic.
Let's use a popular home and garden blog about a home inspection checklist to illustrate an example. If the blog mentions that one of the essential steps of a home inspection is plumbing and drain cleaning, but doesn't provide in-depth details, this is an opportunity to include a link that provides more information to the reader about this topic.
Having a link to a page that explains plumbing and drain cleaning provides the reader with the opportunity to learn more should they require additional information about it.
Inserted links that incorporate a mention to you can either be "follow" or "no-follow" links, depending on the situation. Earning the link insertion through legitimate means is considered white hat and above board.