I received the following email from a colleague, who is a native speaker of American English:
Back on for today. Starting at [10:00].
What does "back on for today" mean?
It could mean that the person is back online and available. It could also mean that a meeting or appointment that was previously cancelled is now going to happen as originally planned. (Or maybe at 10:00 if that wasn't the originally planned time.)
This normally means something was originally scheduled for a certain day, later had to be rescheduled, then finally re-rescheduled for the original day.
For example: Tom, a project manager, schedules (3 weeks ahead of time) a project meeting for a Wednesday afternoon (on Dec 17). Everyone confirms they can attend the meeting. Then 4 days before the meeting (Dec 13), a key participant (John Smith) has an emergency and cannot attend on Tuesday. (so then there is last minute rescheduling of the meeting.) Then on the Monday, Dec. 15, John Smith informs the organizer that there is no longer an emergency so he can now attend the meeting as originally scheduled on Dec 17. (At this point the organizer would contact all the participants and likely say "The meeting is back on for Dec 17".