The word "advent" is from the Latin word for "coming," and as such, describes the "coming" of our Lord Jesus Christ into the flesh.
Advent begins the church year because the church year begins where Jesus' earthly life began--in the Old Testament prophecies of his incarnation. After Advent comes Christmas, which is about his birth; then Epiphany, about his miracles and ministry; then Lent, about his Calvary-bound mission; then Easter, about his resurrection and the sending of the apostles; and then Ascension (40 days after Easter) and Pentecost, with the sending of the Holy Spirit.
The first half of the church year (approximately December through June) highlights the *life* of Christ. The second half (approximately June through November) highlights the *teachings* of Christ. The parables and miracles play a big part here. That's "the church year in a nutshell," and it should help reveal how Advent fits into "the big picture."
Advent specifically focuses on Christ's "coming," but Christ's coming manifests itself among us in three ways--past, present, and future. The readings which highlight Christ's coming in the past focus on the Old Testament prophecies of his incarnation at Bethlehem. The readings which highlight Christ's coming in the future focus on his "second coming" on the Last Day at the end of time. And the readings which highlight Christ's coming in the present focus on his ministry among us through Word and Sacrament today.