Essentially (other posters can provide more technical details), my general recollection is that a Cardinal Humbard had led a small delegation from the Church of Rome to Constantinople, in an effort to reconcile a dispute between the two churches, due to Rome's addition to the 8th Article of the Nicene-Constantinopolen Creed, the Symbol of Faith, with the words "and from the Son," "Filoque," in Latin, which, had been unaltered since it was issued by the First and Second Ecumenical Synods (Councils), and had stated that the Holy Spirit "proceeds from the Father" (only), consistent with the Gospel of Saint John, Chapter 15, Verse 26, "but when the Helper comes, whom I shall send to you from the Father, He will testify of Me." The Undivided Church had held previously that once an Ecumenical Synod had promulgated matters of Faith, and it was accepted by the Church as a whole, and was reconfirmed by a subsequent Ecumenical Synod, it is doctrine that must be believed by the the faithful; and that no one had the authority to alter it. When the local synod in the West had first added this language to the Symbol of Faith, the Pope had rejected their addition of the "Filioque." A Pope had also emblazoned it in silver on the walls of the Vatican, so that it wouldn't be altered. It was Charlemagne, as I recall, who had pressed a subsequent pope to amend the Creed. The Eastern Patriarchs refused to make the change, advising that no one had the authority to alter church doctrine, especially when it has scripture as its basis. After a few meetings with Constantinople's representatives, Cardinal Humbard issued the Anathema against "Patriarch Michael, and all who follow him," as I recall, having placed it upon the Holy Table of the Cathedral of the Holy Wisdom of God (Hagia Sophia), July, 1054.
If you enter a "search" of this topic, there have been lots of discussions of this matter, which remains Orthodoxy's principle dispute with the Church of Rome, as it symbolizes the authority that the Western Church has ascribed to the pope, which Eastern Orthodoxy wholly disputes.