Please see https://en.wikipedia.org/wiki/Atomic_commit#Atomic_commit_convention
Basically it means do not combine unrelated code changes (ex: two different bugs) or a lot of code changes (even when related) into a single commit.
Even combining similar bug fixes (say undefined keys) in a single merge request to review together, rarely makes sense: If any is rejected it blocks the entire MR, and if two lead to discussion is make comment threads very confusing.
It is different but a similar idea to an Atomic Upgrade. Ex.: You can revert both easily.