I think mirroring means what my T often does: she adapts her behaviour to mine. When I'm relaxed, she's too; when I'm silent, she doesn't say a lot either; when I make a joke, she says something funny too; when I gaze at her, she gazes back.
Except, when I'm stressed and upset and crying, she's calm. There must remain some difference...
I guess she does the mirroring thing to make me feel at ease and understood. And it makes me more aware of my own behaviour and how I'm actually feeling than anything else.

That's what I think it means.