Fine, I’ll give some broad definition, but I do not want this thread to go rigorously into the nuances.
The Magisterium is the Church’s teaching authority vested with the Pope and Body of Bishops. For a teaching to be incorporated into the Magisterium, it must be declared and defined as an issue of Faith and Morals in documents that have the capacity to be authoritative (papal bulls, encyclicals, apostolic exhortions, etc).
The infallible Magisterium are those subset of the teachings that have no possibility of error. They are declared as solemn definitions of Faith and Morals at ecumenical councils, when the Pope speaks and invokes the extraordinary Magisterium (ex cathedra), or with other Ordinary teachings. These ordinary teachings are either part of the Ordinary and Universal Magisterium, which require the participation of the Pope and Body of Bishops dispersed throughout the world, or part of the Pope’s ordinary teachings that are also definitive and irreformible.
It is way more nuanced than that. But I do not want this thread to be about the nuances of infallibility.