Anchor text is the visible, clickable part of a link. In the sentence "Margin keeps your sources attached to every note", if the word Margin is linked, then Margin is the anchor text.
Search engines read anchor text as a short description of the page it points to. That makes it useful, and also easy to abuse: a profile where most links use the exact keyword you want to rank for looks engineered, because real people mostly link with your brand name, a URL or a phrase that fits their sentence.
In outreach, let the editor choose the anchor. If you have to suggest one, offer your brand name or a plain description of what you do.
Example
A natural profile for Margin might be mostly "Margin", "margin.example" and "this notes app", with only a few links using "research notes app".
Putting it into practice
Where to use it: Anchor text distribution checker and Listicle inclusion email template.