。
She is humble about her achievements.
Literal
She [topic-は] her own achievements [about/toward-に] humble is.
謙虚 ('humble, modest') is a Sino-Japanese na-adjective for a culturally celebrated trait — the disposition not to boast about one's accomplishments. The に here marks the area of the humility ('humble about / regarding X'). である is the formal/literary copula, equivalent to だ but used in essays, news, and formal description — so the sentence reads like an evaluative observation about her character in print rather than spoken praise.