The Business Overview report is a useful tool for assessing how your business is doing each month.
For starters, you will want to make sure that you have a different account code set for labour and material sales in the Xero Settings - this makes sure the report can tell them apart and prevents odd results.
The contents of this report are drawn from totals in your Xero account - the figures are pulled from the account codes you have turned on in the Assign Accounts section of the Xero Settings, which are used exclusively for this report:
To get the most accurate reporting, you will want to run through these account codes and make sure they are set appropriately. If you are unsure what to set some of your accounts to, your accountant should be able to help you out.