What is a mediatrice?

Perhaps I'm not interpreting you question correctly... Mediatrix of all grace (plural mediatrices) is a title sometimes given to Mary, mother of Jesus, ascribing to her the attribute of being a conduit between God and the people, so that all graces are from God and distributed by her. I don't believe that this is an official teaching of the Roman Catholic church, but it is a cause that may one day become a teaching. This tradition may be part of a movement among some Catholic thinkers to define Mary as having attained the state of "hypostatic union" with the holy trinity, all but equating her with the godhead. Needless to say this is a controversial topic.