。
She squeezed lemon into her tea.
Literal
She [topic-は] black-tea [into-に] lemon [object-を] squeezed.
しぼる ('to squeeze, to wring out') is a tactile, physical verb: squeezing a lemon, wringing out laundry, milking a cow, even squeezing every drop of effort out of someone. The kanji 絞る captures the sense of compression. に on 紅茶 marks the destination of the squeezing — 'into the tea.' Lemon in tea is a Western convention not native to Japanese tea culture, where citrus is more often a side ingredient (柚子湯, citron baths) than a tea additive — but in cafes serving 紅茶, レモンティー ('lemon tea') is a standard menu item.