The episode you are talking about occurs in Book 9 of The Iliad. You can look there for more information.
Basically, the three envoys are coming to Achilles to ask him to help the Achaians, who are losing to the Trojans. Achilles has been angry because Agamemnon took Briseis away from him. Because of this, he has been unwilling to help.
The three envoys promise to give Briseis back along with gifts and an apology. Achilles refuses and says that he will not fight.
As it turns out, he won't join the fight until Patroklos is killed.
He refuses to fight unless the Greeks attack his ships.