What are matte mirrors?
Matte mirrors are opaque mirrors, so you do not see a reflection in them. The front of the glass is etched, in other words made matte.
Because of this matte layer you do not look directly at the silver layer, which is why you see no reflection. The etched layer does let light through. That light reflects off the silver layer and falls back onto the etched layer, which gives the mirror an attractive matte look.
Matte mirrors are also available among the coloured mirrors.
What you can use a matte mirror for, for example as a kitchen splashback, you can read in what to use a matte mirror for.