If you are inputting information for a column that is not acceptable per the rules set up by this customer, then it will trigger validation errors for you inside the platform. When validation errors have been triggered on a row, the specific fields which triggered the error will have a red border and the entire row will be highlighted in red. Once the data is corrected, then these error identifiers will be removed.
You can review this article for more details on Troubleshooting Errors.