An editorial link is one that the person who controls the page decided to add because it's useful for their readers. Nobody paid for it, and it isn't part of a link swap.
These are the links search engines want to reward, and the ones white-hat outreach aims for. Asking an editor to consider your product is fine; the decision, wording and placement stay with them.
A link stops being editorial when money, free products in exchange for a link, or reciprocal arrangements decide whether it appears.
Example
A blogger tries Margin after an outreach email and decides to add it to their list. That's an editorial link.
Putting it into practice
Where to use it: White-hat link building checklist and Declining a paid-link request. For the longer version, read what separates a strong backlink from a weak one.