"In the year that King Uzziah died, I saw the Lord seated on a throne, high and exalted, and the train of his robe filled the temple. Above him were seraphs, each with six wings: With two wings they covered their faces, with two they covered their feet, and with two they were flying. And they were calling to one another: 'Holy, holy, holy is the Lord Almighty; the whole earth is full of his glory.'" NIV translation
God calls to Isaiah in a vision that includes God sitting on his throne in heaven. The train of his robe fills the temple so that there is no room for anything else to be worshiped. Isaiah sees seraphs which are unearthly beings that serve God in heaven. These are very similar if not the same creatures that are described by John in Revelation 4. As in the book of Revelation, the seraphs are proclaiming the holiness of God. Through this vision, God was allowing Isaiah to see that God is set apart (holy) and in charge. A part of that holiness is the fact that He can use whoever He chooses to accomplish His purposes.