In Homer's Iliad, Patroclus is the best friend of Achilles, who is the mightiest of the Greek warriors at Troy. In Iliad 1, Achilles becomes angry at Agamemnon for taking away his female war prize, Briseis, and thus decides to remove himself and his troops (including Patroclus) from the fighting.
After Achilles leaves the battle, the Trojans begin having success against the Greeks. The Trojans' greatest warrior, Hector, goes on a major killing spree, manages to smash his way into the Greek camp, and threatens to set fire to the Greek ships, some of which belong to Achilles and his troops.
Because Achilles was on the verge of leaving Troy and returning home, he allows Patroclus to arm for battle and lead out their troops so that they could drive Hector from the camp and protect their ships. Patroclus would wear Achilles' armor into battle to strike fear into the hearts of Hector and his fellow Trojans. Achilles specifically instructed Patroclus not to do anything other than drive Hector and the Trojans out of the Greek camp.
Unfortunately, Patroclus began having great success once he entered battle and chased the Trojans all the way back to their city. This is where he encountered Hector, who killed Patroclus, stripped off Achilles' armor, and took the armor for his own prize of war.
The death of Patroclus and the loss of his armor upset Achilles' greatly and thus prompted Achilles to return to the fighting so that he could take vengeance upon Hector.
On a deeper level, Achilles' failure to be present when his comrade went into battle illustrates the pain and loss that relentless anger can cause.
Also, on a symbolic level, the loss of Achilles' armor might represent the loss of Achilles' old self. The next time Achilles entered battle he would be wearning new armor, armor crafted by the gods themselves.