。
Dinner's ready.
Literal
Dinner-of preparation [subject-が] become-done-[polite-past-ました].
夕食 ('dinner') is the more neutral/formal of Japanese's several 'dinner' words (vs the homier 夕飯 or the dated 夕御飯). The marking with が (rather than は) gives this a sense of announcing new information — 'Dinner [it is that] is ready!' — rather than returning to an established topic.