。
She looked up at the tall tree.
Literal
She [topic-は] that tall tree [object-を] looked-up-polite.
見上げる is a compound verb fusing 見る ('see, look') + 上げる ('raise'), meaning 'to look up at.' The を marks the object being looked at. Used both literally for tall objects (trees, buildings, mountains) and metaphorically for someone admirable: ~を見上げた人物 ('a person to look up to, an admirable figure'). The polite past ました suggests narrative or careful description; this kind of descriptive sentence often opens children's stories or scenic prose.