Moses commanded the Israelites to only take enough for one day--but some of them didn't listen and took more. The result was that it was rotted and filled with worms. Then, in verse 22, he tells them to gather enough for 2 days, since there will be no manna to gather on the Sabbath. Some of the Israelites didn't gather extra and as a result they were hungry.
My first reaction when I read this is to think, "How dumb could the Israelites be?! They had SPECIFIC instructions from Moses who had specific instructions from God, yet they didn't follow them!" As soon as that thought crossed my mind, I began to think about my own life and how I don't always trust that God will provide what I need when I need it. I want to "store up" my manna "in case God doesn't come through." Or, I don't follow his guidance and neglect prayer, thus missing out of the benefits of gathering manna and storing it up.
The good news is that God has an abundance of resources that we can never deplete. However, he wisely only promises us grace and peace for TODAY. If we were allowed to store up "manna" or "peace" we might begin to feel self sufficient and forget that we truly rely on God for everything.
So, let's be thankful that we have a God who is good enough to provide us with all that we need but wise enough to only give us what we need when we need it. His mercy is perfect!