Cooking with human excrement (human waste) would make the bread ceremonially unclean. This means it would be unfit for worship according to the law of Moses. If Ezekiel ate it, he would become unclean.
This act was a sign of what would happen to the Israelites in exile. In a foreign land, it would be very hard to follow the kosher dietary laws (the food laws God gave to Israel). As a result, they would eat food that was unclean.
Because of this, they would be unclean and separated from the Lord’s cleansing presence (his holy presence that makes people pure).