What does Exodus 7:14 mean?

This begins describing the first of ten plagues God which sends upon Egypt. These are the expected result (Exodus 3:19–20) of Pharaoh's stubbornness. At this stage, Pharaoh's obstinate heart is entirely his own fault. Only after many cycles of resistance (Proverbs 29:1) will the Lord begin making a terrible example of the Egyptian king (Exodus 4:21). Here, as in the other plagues, the catastrophe is directly tied to Pharaoh's refusal to release the Hebrew slaves (Exodus 1:11–14).

The first plague comes with interesting timing. Most of the later disasters aren't given a specific duration. This one will last seven days (Exodus 7:25). The number suggests completion or perfection and emphasizes that this is a divine occurrence. During that time, the Nile river and all the other surface waters of Egypt (Exodus 7:24) will be rancid, undrinkable blood (Exodus 7:17–20).