Essentially, Gulliver's Houyhnhnms host asks Gulliver to tell him everything of the land from which he has come. Gulliver's host is unable to comprehend the world he comes from, where Yahoos (humans) are endowed with reason and Houyhnhnms (horses) are beasts of burden.
My master, after some expressions of great indignation, wondered how we dared to venture upon a Houyhnhnm's back; for he was sure that the weakest servant in his house would be able to shake off the strongest Yahoo; or by lying down and rolling on his back squeeze the brute to death.
Additionally, Gulliver struggles to explain some of the vices inherent to men, such as dishonesty, lust, or envy. The Houyhnhnms are so honest and fundamentally virtuous that they have no equivalent words in their language to describe these very human traits. Additionally, his host does not understand the ideas of religious, political, or especially legal disputes.