Learn Hardcore Indonesian: Rapat besok pagi dibatalkan karena listrik mati. - Tomorrow morning’s meeting was canceled because the electricity went out.

dibatalkan is the passive form of batal (to cancel). In Indonesian, passive voice uses the prefix di- (and here the suffix -kan because batal takes an object). This structure means “the meeting is canceled” (the action happens to rapat).
By contrast, membatalkan is the active transitive verb “to cancel (something),” which would require an explicit subject performing the action (e.g., Manajemen membatalkan rapat).

What does besok pagi mean, and why is it placed right after rapat?

besok pagi means “tomorrow morning.” When you say rapat besok pagi, you’re using besok pagi as a modifier of the noun rapat (“the meeting that’s tomorrow morning”).
If you moved besok pagi to the end—Rapat dibatalkan besok pagi—you’d instead be specifying when the cancellation occurs. Word order in Indonesian is flexible, but placing a time phrase immediately after a noun makes it a noun modifier.

Why isn’t there an explicit subject (agent) who canceled the meeting?

Indonesian often uses an agentless passive to focus on the action or result rather than the doer. Omitting the agent is natural when it’s obvious or unimportant. If you did want to mention who canceled it, you’d add oleh (by):

Rapat dibatalkan oleh manajemen.