They are talking about spot directing, which you can get more information about here. It looks like your layers are correct, so one character should be behind and the other in front of the mirror.
When they are saying to spot the duplicate lower, notice how the forward facing character (in the mirror) is just slightly shorter than the rear facing person. You’ll just want to adjust your y-axis number so that the character is a little lower on the screen.
This is not my picture, I just got it off Google.
Hopefully this answered your question a bit. If not, let me know.