In the biblical account, what does god mean by "plagued Egypt" and why would he do such a thing

The act of God "plaguing" the Egyptians means that he sent numerous plagues, ten to be precise, upon the people of Egypt to bring upon them judgement.

God "plagued Egypt" as a judgement for the king Pharaoh and his people's oppression of the Israelites, which God called "His people".

Explanation:

In the book of Exodus in the Bible, when God, through Moses demanded for the release of the Israelite people from the clutches of Pharaoh, Pharaoh refused. He did not want to let go of the workers who, besides multiplying quickly and bringing more profit to their work, are the main workers for the welfare of his people. So, God brought upon them different means of torture, plagues which are a form of judgement for this very act of refusal.

The ten plagues of God are (Exodus 7- 11) -

Turning water into blood.
Frogs everywhere
Lice or gnats upon everyone and everything.
wild animals that destroy humans and plants.
plague on the livestock
outbreak of boils on man and animals alike
hail and fire
locusts destroying everything
darkness for three days consecutively
death of the firstborn, be it animal or human (except the Israelites).

These plagues were sent to pass judgement on the people of Egypt, and especially to Pharaoh to show the powerful nature of the God of the Israelites. It was also done due to the refusal of Pharaoh in letting the Israelites go out of Egypt to worship the Lord their God.