Most of the time the pope is no different from any other Catholic. He has a lot of respect by virtue of his post, but what he says is not the word of God.

This changes if he speaks “ex cathedra” (Latin for “from the chair”, the throne the previous poster is alluding to). Ex cathedra means the pope is defining dogma for all the faithful as the supreme pontiff of the Catholic Church. Think of it as the difference of the US president saying something in a private conversation vs. him issuing an executive order. When the pope speaks ex cathedra, he is considered infallible in the Catholic faith.

Popes rarely speak ex cathedra. Most of Catholic theology is settled, so there is rarely need to clarify anything. The last time it happened was in 1950 about the Assumption of Mary.

Papal (and church council) infallibility does mean that the Catholic Church can never change its mind about things like homosexual marriage and abortion. The Catholic Church says it is Christ’s church on earth, and is protected by the Holy Spirit from error that could lead Christians astray. Saying that they got something as vital as “what is and is not sin” wrong would undermine the church’s entire foundation.