God tells Ezekiel that, during this time period, he'll eat bread made from a bunch of different grains (the recipe for Ezekiel Bread).
He'll mix it with water to make something like barley cakes and cook it on human dung, eating it through the duration of the time he'll by lying on his sides. (This is the same kind of food people in the siege will be forced to eat, just so you don't think Ezekiel's getting singled out here.)
God specifies the measurements for how much grain and water he can use and how much water he can drink.
But Ezekiel objects to the human dung part. He says that he's always been clean in his eating habits and has never eaten carrion or anything like that.
God relents and allows him to use cow dung (not an uncommon practice, but still) to cook his bread instead.
Then God predicts that Jerusalem will eat bread and drink water in the same meager way during the siege until they run out of grain and starve.