The performance report explained

The Performance tab under Reports is your monthly business health check. Use the arrows at the top to move between months.

The four headline cards

Revenue taken. Money actually received in the month (succeeded payments: deposits, balances, retail and top-ups, less refunds), with a percentage change against the previous month.
Appointments completed, with upcoming and cancelled counts alongside.
No-show rate. The percentage of appointments missed without notice. It turns red above 10%, which is your cue to look at deposits and reminders.
Rebooked. The percentage of patients seen this month who already have a next visit in the diary. This is the single best predictor of next month's revenue.

The detail panels

Revenue breakdown splits the total by type: deposits, balances, retail, account top-ups and refunds.
Patients this month shows how many patients you saw, split into new and returning. A healthy clinic needs both: new patients for growth, returning ones for stability.
Treatment mix shows which treatments made up your month, as a bar per treatment.
By practitioner lists each practitioner's completed appointments and the revenue attributed to them (via the payments linked to their appointments).

Reading it honestly

Revenue here is money taken in the month, not bookings made, so a deposit taken in June for a July appointment counts in June. Nothing needs setting up: the report builds itself from the appointments and payments you already record. The other tabs on the same page cover record completeness and complications.