...a sacrament is a sign that is both a reminder of the past, i.e. the passion of Christ; and an indication of that which is effected in us by Christ's passion, i.e. grace; and a prognostic, that is, a foretelling of future glory.
Each sacrament unites those three points in time: Christ's death on the Cross; the current moment; and the Last Day. We speak in particular of the Eucharist making present Christ's sacrifice on the altar, but we should always recall it as a pledge of future glory as well.
In a similar way, Advent makes present to us the past and the future. The past, in the form of the promises of a Messiah, culminating in the journey to Bethlehem and echoed in the ministry of St. John the Baptist. The future, as we look forward to the Messiah's return.
And the present? What are the graces that indicate of that which is effected in us, by Christ's passion, in terms of these preparations? Answering that question, you might say, is what Advent is for.