。
She showed us a pretty hat.
Literal
She [topic-は] us [to-に] pretty hat [object-を] showed.
見せる ('to show') is the transitive partner of 見える ('to be visible / can see') — Japanese keeps these neatly distinct: 見せる you actively show; 見える just appears in your view. The recipient takes に, the thing shown takes を. きれいな is the attributive form of きれい — a na-adjective often translated as 'pretty / clean / beautiful' depending on context. With a hat, きれい leans toward 'pretty' or 'lovely' rather than 'clean.'