This sets which of the available offers the price rows the rate applies to. If you are showing only one rate leave the default value "Price 1" for all rates.
Change ONLY if If you are using EXTRA PRICES (SETTINGS -> BOOKING PAGE -> ROOMS -> EXTRA PRICES ) to offer not showing multiple prices (e.g. standard rate/non refundable rate, packages). Select which price row offers for your rooms leave this rate should apply set to.offer 1