I'll begin with the disclaimer that I dislike the term "Sea Peoples" for reasons I've outlined here. Unfortunately, it's been used so frequently that I think we're stuck with it.

"Sea Peoples" is a label applied to several groups from various places in the eastern Mediterranean, particularly the Aegean and Anatolia. Several of the groups appear in texts nearly 200 years before the end of the Bronze Age, often allied with the great powers of the Late Bronze Age. In two letters to the king of Egypt (EA 122 and 123) dating to around 1340 BCE, for example, the vassal king of Byblos complained that the Egyptian governor of nearby Kumidi killed a Sherden within his town (presumably a mercenary hired by Byblos).

Paḫuru perpetrated a great misdeed against me. He sent Sutean men, and they killed a Sherden and took three men (as captives) into the land of Egypt. And how many days has the city been enraged at me! And behold, the city is saying, “A deed that has never been done since time immemorial has been done to us!” So send the men lest the city commit rebellion.

There is always more to be said on the Sea Peoples, but I discussed them in these threads:

• Tell me about the Philistines vs others in the Levant 1200 to 800 BCE

• What do we know about the so-called Sea People's origins?

Also see u/kookingpot's answer to Who were the Sea Peoples of the 12th century BC?