To put into simpler words, you must have the base level of your eyes applied with the same set and texture. The Heterochromia in Sims 4 is the dreamy eyes, which is made to, at a time, overlap one eye.
In the real world, Heterochromia refers to the condition when a person has two different eye colours similarly, in Sims 4 heterochromia, the feature allows you to apply the same appearance to your character.
