Agamemnon had angered Achilles, so Achilles was sitting out the Trojan War, but Patroclus tried to talk him into rejoining or, if not, at least to lend him his armor and let him lead the Myrmidons into battle. Achilles agreed to let Patroclus fight dressed in his armor and to lead the Myrmidons.
Patroclus went into battle looking just like Achilles, at least to the Trojans. The Trojans were afraid of Achilles because he was the greatest of the Greeks. Having him sitting out the war was good for the Trojans. Having him back fighting was dangerous. That made the Achilles figure who was secretly Patroclus a prized Trojan target. Although Patroclus wasn't as good a warrior as Achilles, he did kill Sarpedon and many other Trojans.
Patroclus was killed, ultimately, by Hector.
After Achilles got revenge of his friend's killing by killing Hector, he cremated Patroclus' corpse and held elaborate funeral games to honor him.
