Moses has to pour out some water from the Nile. That water will turn into blood on the ground. Blood is a sign of life (Leviticus 17:11), but when water turns into blood, it means death. The Nile is seen as a god who gives life (living water). However, when the Nile will become blood, it will mean death.
Later we read that this sign happened slightly different, because Moses had to stretch out his staff over the waters of Egypt (they all depend on the Nile, and it will be changed into blood. That was the first plague (Exodus 7:14–18).
9 If they will not believe even these two signs or listen to your voice, you shall take some water from the Nile and pour it on the dry ground, and the water that you shall take from the Nile will become blood on the dry ground.”