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:

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'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, [490]
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."