。
She comes from a good family.
Literal
She [topic-は] upbringing [subject-が] is-good.
育ちがよい ('good upbringing, well-bred') is a fixed character description, often implying not just personal manners but also a comfortable family background — the kind of polish that comes from having been raised with care, education, and means. The opposite 育ちが悪い is bluntly insulting. The pattern noun + が + adjective is the quintessential 'X has a Y quality' frame in Japanese, parallel to 性格がいい (good personality), 顔がいい (good-looking).