8 And the Lord spake unto Moses and unto Aaron, saying,

9 When Pharaoh shall speak unto you, saying, Shew a miracle for you: then thou shalt say unto Aaron, Take thy rod, and cast it before Pharaoh, and it shall become a serpent.

Moses had seen the miracle with the snake in Mount Horeb, and had been instructed to show it also to the Israelites, and now he was prepared to do the same for Pharaoh.

God foretold that Pharaoh, himself, would be the one to ask for this sign, presumably as evidence that the Lord had sufficient power to demand that Pharaoh let the Israelites go into the wilderness. Of course, asking for a sign is often treacherous in the Bible. Those who say they will not believe except for a sign, typically do not believe with a sign either, and so God turns the sign into a curse against them. The disbelieving stake their plans on God not being to deliver on His promises, and so He breaks their plans to keep His word. Since they sought to call the Lord’s bluff, their own is called instead. This is, indeed, how things would go for Pharaoh, too. He will, in fact, see many wonders and miracles, but all of them to his own detriment!

Another small detail that other commentators have observed is that Moses says “take thy rod” to Aaron. Assuming that this was the shepherd’s staff that Moses had used in Midian, it would seem Moses had now given it to Aaron.