This is taken from the Help file:-
Select this check box to preserve the existing transaction logs on the Exchange 2000 or 2003 server. Transaction logs from the storage media are then restored and added to the existing set of transaction logs on the Exchange 2000 or 2003 server. When the restore operation finishes, Exchange 2000 or 2003 automatically updates its databases with the uncommitted transactions found in the existing and newly-restored transaction logs. This option is selected by default.
If you are restoring individual databases into a storage group, the No loss restore check box should be selected. If this check box is not selected, uncommitted transactions for other databases in the storage group may be lost.
Put it simply -
1. When No Loss Restore is checked - existing transaction logs will not be deleted. When the restore finishes the logs will be replayed and the server should not have lost any data.
2. When No Loss Restore is unchecked - existing transaction logs will be deleted. Once the restore has finished the exchange stores will be at a point that of the end of the backup you just restored.
In your case because you are restoring to the RSG, you would uncheck No Loss Restore.
There is also a Guide Me Wizard feature in the Exhange settings of the restore job - it will ask you exactly what you want to do and then make the appropriate settings for you.