Two reasons this message might be critical:
--1. When starting out, we highly recommend that you import only one period to ensure that your import field mappings are correct and your matching options/rules are set.
--2. If you will be reconciling periods - it is possible that your transactions are not being classified into the correct/intended period.
You can set the option to display the 'Assign transactions to period based on' feature by going to Matching Rules > Settings > General.
Note: If you are not using Bank Rec's Reconciliation feature, you can ignore the above items and import into multiple periods at one time as well as suppress the message prompt (Matching Rules > Settings > General).