The Shofar 1: "Awake, O You Sleepers"

What is the meaning of the shofar which we blow on Rosh Hashanah? A quotation garnered from S. Y. Agnon's Days of Awe:

Despite the fact that the blowing of the ram's-horn on Rosh ha-Shanah is an explicit decree in the Scripture, it is also an allusion, as if to say: Awake, O you sleepers, awake from your sleep! O you slumberers, awake from your slumber! Search your deeds and turn in Teshuvah. Remember your Creator, O you who forget the truth in the vanities of time and go astray all the year after vanity and folly that neither profit nor save. Look to your souls, and better your ways and actions. Let every one of you abandon his evil way and his wicked thought, which is not good. [Maimonides, Hilkhot Teshuvah III.4]

For more on the past, present, and future meanings of the shofar, from a Messianic Jewish perspective, check out Derek Leman's article Elul: Preparation, Reflection, Spiritual Discipline.