The first Sunday of Advent is the beginning of the Church year. Advent is a time of waiting and preparation for the birth of Jesus Christ on Christmas. This season lasts for roughly four weeks, always beginning four Sundays before December 25th, Christmas Day.
Advent is a time of waiting and preparing in two ways. First, we wait and prepare to celebrate the Nativity of the Lord (a fancy way of saying Jesus' birthday) on Christmas. We also wait and prepare for that moment when Christ comes again in glory, the Second Coming, the Final Judgment.
The color for Advent is purple. This is because it is a time to be a little more subdued and reflective; we have the hope of Jesus on the horizon, but He has not yet come. The whole liturgy reflects this attitude: our Scripture readings, our decorations, even our music points us to this time of hopeful, quiet waiting for Christ.