。
He's known as a great poet.
Literal
He [topic-は] great poet [as-として] is-known.
~として ('as, in the role of') marks identity in a defined capacity — 'as a poet,' 'in the capacity of a poet,' framing the description as how he's perceived publicly rather than as a bare claim about who he is. ~として知られている ('is known as') is the canonical 'famous for being X' frame; the passive 知られる ('is known') reflects the reputation as something held by others about the subject. 偉大な uses な-adjective form before 詩人 ('poet').