。
She is very sad.
Literal
She [topic-は] very grieving-is.
悲しんでいる uses the verb 悲しむ ('to grieve, feel sadness') in its ~ている form — capturing sadness as an active, ongoing process rather than the static adjective 悲しい ('is sad'). The verb form pulls more weight: it portrays someone visibly grieving, while 悲しい is closer to a flat statement of feeling. Japanese has these emotion-verb / emotion-adjective pairs across the board: 喜ぶ/嬉しい, 苦しむ/苦しい, 怖がる/怖い.