Sending Safety is a setting to make sure you are not sending out multiple emails to the same prospect within the timeframe you select.
Once the time set for sending safety has passed, the sequence will automatically send emails to those not received. This setting can be applied to Manual, Bulk, and Sequence emails.
The below screenshot highlights the Sending Safety settings.
If you are receiving an error message regarding sending safety restrictions, this is because the system is preventing sending due to how you have your settings above. To fix, you can simply toggle on or off the different options, and/or adjust the hour/day delay in the upper right-hand corner!