There are five traditional Catholic beliefs about Mary:
The first four of these have been defined as dogmas of the faith (ie, they have been infallibly defined by the Pope or by an Ecumenical Council). There is considerable impetus at the moment for the Holy Father to make an ex cathedra declaration of the infallible truth of the fifth and final Marian dogma.
There is a lay movement called Vox Populi Mariae Mediatrici promoting this doctrine and setting in train the processes to bring about this proclamation. With the Immaculate Conception and the Assumption, the procedure was that following theological foundations of the doctrines and episcopal support, there had to be a manifestation of the sensum fidelium, ie, that the faithful believed it to be true. Eight million of the faithful petitioned the Holy Father over 95 years for the definition of the Assumption. So far, in just four years, four million signatures have been gathered for the definition of Our Lady as Mediatrix of All Graces.
The doctrine, essentially, is that as a special privilege and reward for her perfection, all Graces that Jesus, as the one Mediator between the Father and Man, obtains for us, are dispensed by and through the Virgin Mary, Queen of Heaven.
St Maximilian Kolbe reflected and wrote extensively on Mary as Mediatrix of Graces.