Memento, homo, quia pulvis es, et in pulverem reverteris.
In English, the italicized text reads: "Remember, O man, that thou art dust, and unto dust thou shalt return."
You are dust. It isn't a pleasant thought. In fact, it's downright scary. These are the last words the Lord spoke to Adam before evicting him from the Garden of Eden, as recorded in Genesis 3 (v. 19). Man is dust, shall return to dust, and no longer has part in eternity. He is restricted from paradise.
These words of the Sacred Scriptures also comprise the epitome of the Imposition of Ashes, the opening Rite from whence Ash Wednesday gets its name. As most readers will probably know, Ash Wednesday is the beginning of the season of Lent, a 40-day span (exclusive of the Sundays!) that historically marks a period of fasting and repentance in imitation of the 40 days our Lord spent in the wilderness. The season is also marked by an emphasis on the Lord's Passion (that is, His suffering and death as payment for the sins of the world in order that He might free us from the bonds of Satan and his kingdom). This especially occurs during the latter weeks of Lent, which are specifically known as "Passiontide."
To this end, Lent is a season of doleful repentance over the sin - our sin - which caused our Savior to suffer. Ashes in particular have long been associated with this theme of repentance. Using one of the most potent examples, one of the collects for the Imposition Rite recalls how the Lord granted "healing pardon to the Ninevites during their penance in sackcloth and ashes." The blessed prophet Jonah reminds us that the great city of Nineveh, sinful as it was, received pardon from the Lord because they repented of their sins and put faith in the hope of His mercy.
When a penitent comes forward to receive ashes on his forehead and hears the words of the Lord: "thou are dust and unto dust thou shalt return," it is hard to believe that God is a God "Who desires not the death, but the repentance of sinners," in the words of another one of the collects, borrowed themselves from the holy prophet Ezekiel. As we meditate on the Passion of our Lord, it is easy to get lost in feelings of guilt and remorse, not believing that God could care about something so insignificant as "dust."
It is this context that makes those words from Genesis so beautiful. Man is dust and is returning to the same. Dust has no power. It blows in the wind. But Christ not only cares about the dust that is man; He took the dust upon Himself. He became dust, making Himself like unto nothing. As terrifying as Genesis' reminder that we are dust can be, it also alludes to the entirety and necessity of Christ's mission. Christ became dust for us and returned to it. He took the death that was intended for us and made it His own. And in so doing, as the fullness of the Deity, He swallowed it. Death is defeated. Where is its sting? Where is its victory? We may return to dust, but Christ will return to us. He will raise us on the Last Day; with our own eyes we will see God. And on that Day of His glorious Parousia, the Second Adam will take us back into the paradise lost by the first.