What does Odysseus learn from his conversation with Achilles?
This depends on which conversation with Achilles you mean. Achilles and Odysseus have two major encounters/conversations.
In Book 9 of Homer's Iliad, Odysseus is part of the embassy sent by Agamemnon to persuade Achilles to return to battle. In this encounter, Odysseus learns that Achilles has no intention of coming back to the battle, despite all of the fabulous gifts that Agamemon is offering.
Odysseus also encounters the spirit of Achilles in Odyssey 11 when Odysseus travels to ends of the world and conjures up spirits from the underworld. In this encounter, Odysseus hears something very significant about life after death, namely that it is a miserable existence. Ian Johnston translates Achilles' famous comment to Odysseus as follows:
'Don't try to comfort me about my death,
glorious Odysseus. I'd rather live
working as a wage-labourer for hire
by some other man, one who had no land
and not much in the way of livelihood, 
than lord it over all the wasted dead.
Thus, from Achilles' perspective, it would be better to be the slave of a poor man on earth than to be king of the underworld.
This quotation was later used by John Milton in Paradise Lost, when he has Satan say, "Better to reign in Hell than to serve in Heaven."
In Book 11, Odysseus meets Achilles in the underworld and hears about Peleus, Achilles's father. Odysseus learns that "old age has lamed his [Peleus's] arms and legs," and that rivals are threatening Peleus. Achilles wishes that he could be on earth among the living to help his father. In this way, Achilles's story is similar to that of Odysseus, whose home is threatened by the suitors while he is away. Odysseus is also learns that Achilles would "rather slave on earth for another man" than be a celebrated hero in the afterlife. In other words, life in the underworld as a lauded figure still does not compare to living on earth as a slave. Odysseus learns that life is worth fighting for and that the underworld offers few rewards, even if one is celebrated. The episode reminds Odysseus of his desire to return home.