Here, Odysseus describes the cyclops named Polyphemus, who is a son of Poseidon, the god of the sea. Of this monstrous creature, he says, to paraphrase:
This ponderously big monster man sleeps in his cave all by himself, taking his flocks of sheep to graze in a field far away from any of his fellow cyclopes, and this alienation means that the brute keeps his own, savage ways. He is so savage and different that he hardly seems like a regular man at all but, rather, like a solitary and boorish mountain.
He uses a metaphor, a comparison of two unalike things where one thing is said to be another, to compare the cyclops to a mountain, emphasizing his size and craggy appearance. Odysseus seems to want to establish the cyclops, right away, as different from him, his crew, and his audience. He seems to desire to prejudice his audience against Polyphemus in order to present him in as savage and unsympathetic a way as possible.