Monthly rate = what you agreed with the client per month. This is set in the client profile and is automatically included in the revenue calculation for every month the client is active.
Actual revenue = what you actually invoiced or received in that month. This may differ for one-off extra-work invoices, partial months, discounts or bonuses.
Is there a difference? Then use a monthly override (Overview → pencil icon on the month) so your reporting shows the actual numbers.