An exact-match anchor uses precisely the search term you're trying to rank for. If Margin wants to rank for "research notes app", a link with that exact text is exact-match.
A few of these appear naturally. But when a large share of a site's links use the same commercial keyword, it signals that the links were arranged rather than earned, which is exactly what Google's link spam policies target.
Don't ask editors for exact-match anchors. Let them link your brand name or describe you in their own words.
Example
If 40% of Margin's links said "best research notes app", that would be a red flag.
Putting it into practice
Where to use it: Anchor text distribution checker.