If you want to do a serious research project on this question, you would start with the Wikipedia article on Advent, which is actually a pretty good introduction.
Advent is a season in the church year, a time to spiritually prepare for Christ coming into the world. It varies in length, because there are always four Sundays in Advent, and Christmas, being a fixed date, can be any day of the week. (By my calculations, this year, because December 25th falls on a Saturday, the Advent season is one day less than the longest it could possibly be -- if Christmas fell on a Sunday.)
Advent also marks the beginning of the church year and the lectionary cycle.
The gospel text for Sunday (yesterday) is about the second coming of Christ. The key verses are Matthew 24:42 and 24:44: "Keep awake therefore, for you do not know on what day your Lord is coming." and "Therefore you also must be ready, for the Son of Man is coming at an unexpected hour." Every year, the beginning of the church year is marked by looking forward to the end (yes, The End), which, in the Biblical story, is the return of Christ. It's always interesting to me the way a very long timeline lines up with a yearly cycle -- in order to prepare our hearts (once again, this year) for the coming of Christ (in the manger), we are encouraged to PREPARE for the Second Coming of Christ. We look forward to the coming of Christ in a unique way at the same time as we prepare to welcome the child in the manger according to the cyclical remembrance.
Perhaps the most important part of this text and the larger Advent focus is the repetition of admonitions -- Keep Awake, Be Ready, Prepare. Because Light, Love, Redemption and Salvation both came in a unique way, on a long timeline, and reappear in our lives in a cyclical way. So we need to be awake, ready, prepared for Christ to appear -- in the face of a stranger, the love of a friend, a transformational glimpse of beauty, a new challenge that brings us life, or an ordinary miracle, like the birth of a new baby.
I hope that I am ready, and can prepare my heart this Advent and beyond, to receive the Light wherever it appears.
In La Misión Cristiana, the church we are working with here, as far as I know, Advent is not an important season in their church year. They don't follow the traditional Western liturgical calendar (and I hope to learn more about the theological, historical, and cultural reasons behind that). I do, however, see them as open, awake, prepared, ready to see and respond to the ways that God enters the world. Some of those ways are familiar to me, and others less so. I expect that this will be just one of many ways that we will learn more from these brothers and sisters than we could ever hope to teach.