After the death of Aharon, the people complained once again to Moshe Rabbeinu for taking them out of Egypt. They complained about the lack of bread and water and they were tired of the Mann.
The Rabbis say that such behavior demonstrated that they were כפויי טובה, ingrates. They described the מן as לחם הקלוקל, insubstantial bread.
The תורה תמימה explains why they were ingrates. The מן was absorbed completely in the body with no need to eliminate waste. There was no need for them to walk a great distance outside of the camp to relieve themselves. They belittled this חסד that Hashem did for them.
Their punishment for such ingratitude were the נחשים השרפים, the fiery serpents. We must never be ungrateful for the kindnesses Hashem sends our way.