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A woman whose husband has died is called a widow.
Literal
Husband [by-に] died-on woman [topic-は] widow [quotative-と] is-called.
夫に死なれた is the adversative passive: literally 'had her husband die on her,' framing the death as something inflicted on the wife — a Japanese-specific way to convey suffering or unwanted impact. ~と呼ばれる = 'is called.' 未亡人 = 'widow,' literally 'not-yet-deceased person,' a sober euphemism rooted in classical usage.