Under "Statistics > Revenue" you'll find two kind of ways to measure the revenue for a period.
1) The "Value of bookings" column under "Revenue" only measures the value of the existing bookings, not the received amount.
2) The "Cash invoice" and "Paid invoices" columns under Revenue measures the actual amount of money received during that period.
Example: Your hotel is open from May to Sept. During February you receive bookings for the summer together with full prepayments of a value of $10.000. All this bookings have check in and check out during high season in August.
In this example your "Value of bookings" during February will be $0 as you have no rooms booked during that period. But the revenue of "Cash invoices" and "Paid invoices" will be $10.000 during February.
In the same example your "Value of booking" during month of August will be $10.000. But as the bookings during August was fully prepaid in February, the columns "cash invoices" and "invoices" will be $0 for this period.