Magnum Principium is the name of the Motu Propitio released by Pope Francis today. It will take effect October 1, 2017. Rorate Caeli has an excellent analysis so I don't need to reinvent that wheel. In essence though, this papal directive will ascribe to local bishops' conferences the authority to amend liturgical texts and actions based on local proclivities. Now just think of the wide range of differences that exist with the conferences' varying interpretations of Amoralis Lamentia; also consider that in the case of AL, the interpretations that most widely diverge from Sacred Tradition are the ones validated by this pope.
One Peter Five has an analysis, too. In that one, this MP is contrasted to Quo Primum by Pope St. Pius V. The latter was written to unify the liturgies of all the Church into one text and format.
I can only surmise that this Motu Propitio is just the latest deliberated step in the progressives' ultimate goal to undermine the papacy. Recall that Our Lord called Peter "the rock on which I'll build my church". If the rock should crack that would seriously compromise the integrity of the Church. Our Lord promised that the gates of hell would not prevail against the church, but He never promised that the church would be unscathed. In fact, while the Church would survive (in one fashion or another) it could be so battered as to be unable to attend to millions of souls.
Another troubling aspect of potential liturgy-tampering may be found in the phrase "lex orandi lex credendi lex vivandi". In other words, as you pray you believe, and as you believe you live.
UPDATE - Another phrase to describe the anticipated results of this motu is Liturgical Anarchy.