They were Indo-European semi-nomads who migrated to Greece looking for fertile land to grow food. They lived in the Bronze Age. Upon entering Greece, they came across the Pelagius who lived in the Stone Age.
They suppressed the Pelagians, occupied their fertile land and created the Mycenaean civilization. After a while, they came across the Cretan civilization and then became known as the opponents of the Trojans in the war.
The Achaeans then came to be called Greeks, they were strong traders and subjugated the island of Crete, where later they also dominated economically almost the entire Eastern Mediterranean.
Around 1100 BC, the Mycenaean civilization went into decline and then the Bronze Age in Greece ended, giving way to the Dark Ages that lasted about 150 years.