。
It's already acknowledged that there's a meeting tomorrow.
Literal
Tomorrow meeting [subject-が] exist [quotative-ということ] [topic-は] understood-completed-is.
了解済み ('understood and done') uses the prefix 了解 ('comprehension, understanding') + 済み ('completed') — formal business/written register. The close synonym 了承済み conveys the same idea with slightly more deferential overtones; in business contexts the two are often interchangeable, with 了承 a shade more polite when acknowledging a request and 了解 a shade more procedural.