Use the noun manna when you talk about the miraculous supply of food that the Bible describes God providing. In Exodus, the desperate Israelites received manna just when they most needed it.
While the meaning of manna comes from the Old Testament of the Bible, you can use it to describe something unexpected, especially if it feels like a miracle. You might, for example, say, "The doughnuts my coworker brought in this morning were like manna from heaven." Manna has a Greek root that comes from the Hebrew man, and although it literally means "substance exuded by the tamarisk tree," it's almost always used to refer to God's nourishment in the Bible.