Aaron's rod (in some translations, Aaron's staff) refers to the rod which, in the hands of Aaron, Moses' brother, was endowed with miraculous power during the several Egyptian plagues that preceded the Exodus, in the Old Testament of the Bible.
Aaron's rod was important when God wanted to stop the murmuring of the people against the leadership. The head of each tribe offered a rod, which was placed along with Aaron's rod, and the next day, Aaron's rod had budded (Numbers 17:1-13 ). This was to stop murmuring, and the rod was placed inside the ark as a memorial to the miracle. Hebrews mentions the contents of the ark of the covenant, and specifies Aaron's rod (Hebrews 9:4 )
"The LORD said to Moses and Aaron, “When Pharaoh says to you, ‘Perform a miracle,’ then say to Aaron, ‘Take your staff and throw it down before Pharaoh,’ and it will become a snake.” So Moses and Aaron went to Pharaoh and did just as the LORD commanded. Aaron threw his staff down in front of Pharaoh and his officials, and it became a snake. Pharaoh then summoned wise men and sorcerers, and the Egyptian magicians also did the same things by their secret arts: Each one threw down his staff and it became a snake. But Aaron’s staff swallowed up their staffs. Yet Pharaoh’s heart became hard and he would not listen to them, just as the LORD had said." - Exodus 7:8-13