Many were murmurring against Moses and Aaron because they wanted different leaders. They forgot that God Himself appointed these two men, and for murmurring against them, they were rebelling against God. So, the LORD told Moses Moses to collect a rod from each tribe, and the Levites would be represented by Aaron. The next day only Aaron’s rod budded and bore fruit (almonds). This showed everyone (in a peaceful way) that Aaron was God’s choice. God, in His mercy, chose this method so that people would not rebel and die.
When we murmur against our leaders, we rebel against God! In our country we can elect our leaders. If we don’t like them, or if they do a bad job, we can attempt to vote them out, but while in office they are God’s appointed leaders.