for more information on the billing process, see here

Meters log various events that can cause the data to become invalid, such as a Tamper event or Reverse Run.
When the systems reads a meter and an invalidating event comes in, all the data for that period is also marked as invalid.
The validation screen deals with validating data.
The first screen is just a filter, but a blank submit will show all your meters.
Each meter with invalid data is shown per line.
First column is a quick link to the meter.
The problems column is a comma separated list of short codes of the events that caused the data to become invalid.
You can click on this link to take you to the meter's events. (You can also access this screen by selecting the meter and going to View->Meter Event Log). You can hover your mouse over the short codes to get the full description in Meter Event Log.
You can control which events cause a meter's data to become invalid by going to Edit->Meter Event Setup.
The From - To column is a quick link to the View->Profile Graph screen for the time that there is invalid data.
The Last column contains 2 buttons to validate the data. The green one starts the validation process. The yellow one does the same, but logs a message in the form of a Fault Log against that meter (which you can access from Tools->Fault Log).

The validation process consists of 2 steps.
Step 1) Data audit:
There are 2 ways to calculate monthly consumption. A) End Total - Start Total B) Sum of Half Hour Profile Values
Both ways should come to exactly the same value. If they don't, you have an audit error.
You can control the percentage threshold within which the profile and totals has to match up, by changing the Validate within % input.
If a meter fails the audit error, it is shown as a red error message at the top of the table.
You can resolve Audit Errors under Edit->Meter Totals here.
Step 2) Mark all Data as Valid.

Typically you don't just want to mark all data as valid.
I found the quickest way to see if the data is correct is to look at it in the Profile Graph (4th column).