。
She pressed her lips tightly together.
Literal
She [topic-は] mouth [object-を] tightly tied.
口を結ぶ ('to tie/bind one's mouth') is a vivid metaphor for keeping silent — pressing lips firmly closed against speech. The same verb 結ぶ ('to tie, bind, conclude') applies to ribbons (リボンを結ぶ), promises (約束を結ぶ), and friendships (友情を結ぶ). 堅く is the adverbial form of 堅い ('hard, firm') — same kanji as in 心が堅い ('stoic'), 表情が堅い ('stiff expression'). The sentence captures stubborn silence or determination, a body-language detail rich with potential context.