Reader: Is it required to cover the bread at the Shabbos table? And if so why?
Mesora: We cover the bread to mimic the Manna which was provided for the Jews in the desert for 40 years. They had no other sustenance. Each day, only enough for that day appeared. On Friday, a double portion appeared to take them through shabbos too. Since it was covered by a layer of dew, as a protective method of delivering the Manna from God to the Jews, we cover the challah to recall God's kindness in providing the Manna, and we do this on Shabbos as this is an additional lesson that there was twice as a much Manna on Friday.
See this article: 40 Years and the Manna