A Marian Cross is a term to describe a symbolic representation of the close connection of Mary, with the redemptive mission of Jesus.
In that design the letter M is surmounted by a normal Latin cross standing on a bar interlaced with the letter M.[1][2] The papal coat of arms of John Paul II features a cross shifted away from its usual central position to make room for a letter "M" in the sinister base quarter (lower right as seen by the viewer), which represents the Virgin Mary’s presence at Jesus’ death on the cross.
In a 1978 article, Vatican newspaper, L'Osservatore Romano, reported:The coat of arms for Pope John Paul II is intended to be a homage to the central mystery of Christianity, that of Redemption.
The reason for the unusual shift of the vertical part of the cross is striking, if one considers the second object included in the Coat of Arms: the large and majestic capital M, which recalls the presence of the Madonna under the Cross and Her exceptional participation in Redemption.
The Pontiff’s intense devotion to the Holy Virgin is manifested in this manner.