Isaiah is one of the major prophets, along with Jeremiah and Ezekiel. When someone thinks of a prophet and the kind of things a prophet tends to write and say, Isaiah is liable to spring immediately to mind. He's famous for his prophecies about future events—specifically, the final reign of peace on the Holy Mountain.
But is a prophet just someone who predicts the future? That's definitely something prophets do (though if people mend their ways, there's always a chance the negative predictions won't happen), but it's probably fair to say that the main duty of a prophet is to give guidance. Kings and royal officials are all caught up in their usual political concerns and in the difficulties of running a kingdom. That's fine, but they continually run the risk of making running a kingdom and doing business the center of their concern.
Here's where the prophet steps in and helps recall the rulers—and the people as a whole—to their original mission: following the laws of God. He helps replace their mundane, everyday concerns with a bigger and more important concern: obeying the ways of the divine. The prophet, and the prophecies he or she gives, thus help to remind people of where they came from, who they are, and where they're going—keeping their eyes on the prize (i.e., the Kingdom of God) all the while. He keeps calling everyone back to what they know is right, reminding them the outward shows of piety and religiosity aren't good enough. They need to be backed up with a spirit of true devotion and with a commitment to justice, like making sure that widows and orphans are being provided for. The prophet keeps the egotistical tendencies of human beings in check by acquainting them with God.