The staff becomes a snake. In many countries (including Egypt) people were able to freeze
a snake (through snake incantation), which made the snake look like a staff. But here the staff becomes a (dangerous) snake, and Moses has to take the snake by the tail, which is dangerous. The snake is also the symbol for the power of Egypt and the Pharaoh. Pharaoh often had something like a crown which was in the form of a snake. But God has power over dangerous animals and Pharaoh. If God can do this with a staff (the staff of God! Exodus 4:20), he can also move living people.
2 The LORD said to him, “What is that in your hand?” He said, “A staff.”