In Exodus 16 the Israelites complain about how there is nothing to eat out in the wilderness, and God provides them "bread from heaven". This bread is called "Manna".
The word manna comes from the Hebrew text: מן הוא or man hu. This literally means "What is it?" It is a reference to the Israelites not knowing what this flaky substance that came with the dew was really supposed to be.
The name stuck and so when we refer to this divine gift to the starving Israelites, we remember their confused cry of "what is it?"