。
She prepared light snacks for us.
Literal
She [topic-は] us [to-に] light-meal [object-を] prepared for-us.
軽食 (けいしょく, 'light meal, snack') is a Sino-Japanese compound (軽 'light' + 食 'meal/eating') — slightly more formal than the casual おやつ ('snack between meals'). 用意する ('to prepare, to get ready') is a common する-verb for setting things up beforehand: 食事の用意 ('meal preparation'), 用意ができた ('ready'). With ~てくれる, the action becomes a kind gesture toward the speaker's group.