。
She saw a tall man yesterday.
Literal
She [topic-は] yesterday tall man [object-を] saw.
背の高い ('tall,' literally 'whose height is high') is a relative clause functioning attributively — modifying 男性. 背 means 'one's height, stature' and forms a body-part-attribution idiom with 高い: 背が高い ('is tall') in predicate position becomes 背の高い ('whose height is high') when attributing the trait to a noun. The の-が swap for relative clauses is a basic but easy-to-miss point of Japanese grammar — within a noun-modifying clause, the subject can be marked with either が or の, and の often sounds smoother for short clauses. The polite past ~ました places the sentence in everyday-polite register.