An editorial link is a backlink to your site that occurs naturally. A lot of links are gained by asking for them through the act of link building, but editorial links (or organic links) are given without a prompt.
Backlinks to your site, editorial or otherwise, will only come to you if you spend a lot of effort creating valuable content that people are proud to share. Links don't ever just happen, people always need a good reason to link.
Editorial links are always more likely to happen as a result of a good link building campaign. If you've reached out to a number of influencers who were happy to promote your content, their followers will be more likely to consider your content when linking from their sites or social channels.
In Moz's Beginner's guide to link building they describe editorial links as "the holy grail for SEO". This is because Google values editorial links far more than any others – despite the fact it's almost impossible for the search engines to distinguish between the different types of links.