Psalm 135:8, "Who struck the firstborn of Egypt, both of man and animal," refers to the tenth and final plague that God inflicted upon Egypt in the Book of Exodus (specifically, Exodus 11-12).

Here's a breakdown of what it means:

"Who struck the firstborn of Egypt": This refers to God's direct action in causing the death of all the firstborn in Egyptian households. This was the most devastating of the ten plagues.

"both of man and animal": This specifies that the plague affected not only the firstborn sons of Egyptian families but also the firstborn of their livestock. This highlights the comprehensive and devastating nature of the plague. It underscores that it was not merely a judgment against the humans but against their entire economy and way of life, which was heavily dependent on agriculture and livestock.

Significance and Context:

God's Power and Judgment: The verse emphasizes God's immense power and His judgment against Egypt for their oppression and refusal to release the Israelites from slavery.
The Climax of the Plagues: The death of the firstborn was the final and decisive plague. It finally broke Pharaoh's resistance and compelled him to allow the Israelites to leave Egypt.
Passover Connection: The tenth plague led to the institution of the Passover. The Israelites were instructed to sacrifice a lamb and spread its blood on their doorposts, so that the angel of death would "pass over" their houses and spare their firstborn. This event is central to Jewish history and theology, commemorating God's deliverance of the Israelites from slavery.
Remembering God's Actions: As part of Psalm 135, this verse serves as a reminder to the Israelites (and to all who read it) of God's faithfulness and His intervention in their history. It encourages worship and praise for God's mighty acts.

In summary, Psalm 135:8 recalls the catastrophic final plague on Egypt, the death of the firstborn, as a demonstration of God's power, judgment, and ultimately, his redemptive action on behalf of his people.