。
She opened the box's lid.
Literal
She [topic-は] box's lid [object-を] opened.
開ける is the transitive 'open (something).' Note the contrast with intransitive 開く ('open [by itself]') — the same kanji 開 is read 'a-' in 開ける/開く but 'hira-' in 開く ('to spread open'), 'kai-' in 開始 ('opening, beginning'). Same character, multiple readings depending on the form.