。
She met a tall man yesterday.
Literal
She [topic-は] yesterday back [genitive-の] tall man [genitive-の] person [with-に] met.
Japanese expresses 'tall' with 背が高い — literally 'the back/stature is high' — using 背 (back, height) as the subject, not the person themselves. Inside the modifying clause here, that 背が becomes 背の (背の高い男の人), because the が that marks the subject of a relative or attributive clause routinely shifts to の in shorter modifying clauses. 男の人 ('man') is gentler and more polite than the bare 男, which can sound clipped or brusque in everyday speech. 会う takes its meeting-partner with に, treating the encounter as one-sided contact rather than mutual rendezvous (which would use と).