Who were the Philistines in the Bible

The Philistines were an enemy to the people of Israel. They were recorded to have occupied part of the promised land along the Egyptian boarders.
The conflict between the Israelites and the Philistines lasted for centuries until the reign of David who managed to subdue them significantly.

Wail, O gate! Howl, O city! Melt away, all you Philistines! A cloud of smoke comes from the north, and there is not a straggler in its ranks. ISAIAH 14:31

From the time of the judges and lasting for the duration of the monarchy, the Philistines were a thorn in Israel’s side. They had settled in the Promised Land long before God’s chosen people arrived; but like the Israelites, they started out as immigrants from a distant land.
The Philistines from which we get the word Palestine were also known as the Sea Peoples. It is believed they originated from the Aegean Sea region, which comprises mainland Greece, Turkey, and the islands in between. Being accustomed to the sea, the Philistines settled the coastal region of the Promised Land, from the border with Egypt in the south to Gaza in the north.
On their way into the Promised Land, the Israelites took the longer route in order to avoid “Philistine country” (Exodus 13:17). In an allusion to what was to come, the writer of Exodus noted that the Israelites were not yet prepared for war.
Eventually war was what they got. Having failed to subdue the Philistines during the conquest, the Israelites often found themselves oppressed by the Philistines during the time of the judges. The Philistine threat was undoubtedly a factor in Israel’s desire for a king. After three centuries of conflict, the Israelites wanted a strong leader to fight for them (see 1 Samuel 8:20). Their first choice, Saul, proved largely ineffective—he was, in fact, eventually killed in battle with the Philistines. However, Saul’s successor, King David, managed to subdue the Philistines during his reign.
Before he was king, David lived under the protection of Achish, the Philistine ruler of Gath. David was very nearly pressed into service to fight against his fellow Israelites, but at the last minute, Achish
changed his mind on the advice of his military commanders, who did not trust David (see 1 Samuel 28:1–2, 29).