Let’s see what the Catechism has to say about the Magisterium. This is a bit lengthy, but worth reading:
"The task of giving an authentic interpretation of the Word of God, whether in its written form or in the form of Tradition, has been entrusted to the living teaching office of the Church alone. Its authority in this matter is exercised in the name of Jesus Christ." This means that the task of interpretation has been entrusted to the bishops in communion with the successor of Peter, the Bishop of Rome.” (85 CCC)So what does all that mean in a nutshell? It means the Magisterium is the pope and the bishops in communion with him and that it’s the Magisterium’s job to clarify, interpret, and defend all Tradition from incorrect interpretations. The Magisterium was sanctioned by Christ and is our Church’s way of ensuring that the “Word of God, whether in its written or unwritten form, is understood correctly.
"Yet this Magisterium is not superior to the Word of God, but is its servant. It teaches only what has been handed on to it. At the divine command and with the help of the Holy Spirit, it listens to this devotedly, guards it with dedication and
expounds it faithfully. All that it proposes for belief as being divinely revealed is drawn from this single deposit of faith.” (86 CCC)
“Mindful of Christ's words to his apostles: "He who hears you, hears me",49 the faithful receive with docility the teachings and directives that their pastors give them in different forms.” (87 CCC)
The Magisterium has very practical applications. This means that I cannot just pray to the Holy Spirit, read the Bible, and interpret what I read as I see fit. This means that there is an authoritative body, setup by Christ, who has the sole authority to interpret the Bible.
Many would say this is restrictive. They would say that a bunch of bishops can’t tell me what the Bible says. The problem with that line of thinking is that, well, yes they can.
It was the same group of bishops who picked which books went into the Bible and which did not at the end of the fourth century. They had the authority to do that; so it’s only natural that they would have the authority to interpret the books that they determined were inspired.
Keep in mind that both the determination of books and the ongoing interpretation of them, and unwritten tradition, too, is guided by the Holy Spirit. As paragraph 86 above says: “Yet this Magisterium is not superior to the Word of God, but is its servant” The Magisterium may not invent new interpretations; but it has the responsibility to defend the interpretations of the earliest Christians that have been handed down.
The next time someone asks you why you interpret a Bible passage with a Catholic understanding, you may confidently say that you do so because “the very people who put that passage in the Bible interpreted it that way!” We have the Magisterium to thank for keeping us on course and true to our roots.