A smart mirror is a display behind a two-way mirrored surface, so information appears over the viewer's reflection. In retail it turns up in fitting rooms and beauty counters; elsewhere in gyms, hotel bathrooms and lobbies.
The retail case is genuinely good when it solves the fitting-room problem: requesting a different size without getting dressed, seeing whether an item exists in another colour, or calling staff without opening the door. Those are real frictions with measurable conversion attached.
The caveat is that a mirror is a private space, and putting a camera in one is where these projects go wrong. Virtual try-on demands a camera pointed at a partially dressed customer, which is a privacy problem no consent notice on the door fully solves, and a data-protection exposure with very little upside. The versions that survive are the ones that add information without watching.
Also: brightness. A display seen through a half-silvered mirror loses a large share of its output, so panels need to be far brighter than a normal screen, and the room lighting has to be controlled, or the reflection wins.