He isn't talking about God's physical back, but His goodness, as I already told you.
God is telling Moses that He will go before them as they journey through the land. When the nations see God's goodness in leading them with a pillar of cloud and a pillar of fire, feeding them manna from heaven, and giving them water from a rock, Moses will see God and His glory. When God goes before you, you see His "back"....because you follow where He leads. Get it?
Exodus 33:18-19 And he said, I beseech thee, shew me thy glory. And he said, I will make all my goodness pass before thee, and I will proclaim the name of the Lord before thee; and will be gracious to whom I will be gracious, and will shew mercy on whom I will shew mercy.
God shielded Moses from the brightness of His full glory which would have killed him on the spot. That's God's goodness, wouldn't you say?