You can approve or reject a report. You need to have the 'Approver' rights. Then you will see the 'Approve' button on top.
If you open a report from the 'Report Overview' underneath 'Approve', you will see the following:
You will see the buttons: 'Reject' and 'Approve'
Check the expenses to see if they have all the (correct) information.
If the report and expenses are correct, you can 'Approve' the report.
If information is missing or if the information is not correct (VAT, category, amount etc.) you can 'Reject' the report. It is mandatory to fill in a motivation why you are rejecting the report.
Be aware: you always reject the full report, not just one expense!