It's already understood that there's a meeting tomorrow.

Literal

Tomorrow meeting [subject-が] exist [quotative-ということ] [topic-は] acknowledged-completed-is.

了承済み ('acknowledged and done') uses the prefix 了承 ('acknowledgment') + 済み ('completed') — a formal compound meaning 'already accepted/understood.' ということは nominalizes the content clause. Formal, business-register language.