The email re-trigger delay sets the minimum frequency of sending the same email to the same recipient. If you set up a delay for 10 days the email would not be sent until a minimum of 10 days after the previous email (assuming the rule condition is still met at this time).
Natero system runs every few hours within a day to check the trigger conditions taking into account the rule conditions and the re-trigger delay. If an email fails to be sent as it doesn't pass either the rule condition or the retrigger delay, it will be sent as soon as both the rule condition and re-trigger delay are met.
For example, if the retrigger delay is set to 10 days, and on day 1 the email meets the rule conditions and has not been previously sent, then the email will be sent. If 10 days later the rule condition is still met the email will be sent again as it meets the rule condition and the retrigger delay has passed.
Natero does not queue messages until the retrigger delay passes. Both the rule condition and retrigger condition must be met in order to send the message at the time of evaluation.