Scholars have speculated for centuries on the identity of the mysterious "manna," the substance that God sends down for the Israelites to eat in the desert. All you need to know is that manna is directly linked with God's power. Surprise surprise—just like every other symbol in the text.
Manna goes bad if you taste it when God told you not to, and it spoils if you keep it longer than God told you to. Why all the crazy regulations? Well, the manna is meant to demonstrate God's power and mercy at the same time. Sure, he'll provide the hungry with food in the desert, but certain rules have to be followed. It's God's way or the highway.