Transactional emails are emails that are sent when a particular action occurs triggering an email to send. Ex. a purchase is made triggering an order confirmation email, or a shipment is sent triggering a shipping confirmation email to send.
Here is a list of the notification emails you can customize with Spently:
Draft Order Invoice
Gift Card Created
Shipment out for Delivery
Customer Account Invite
Customer Account Welcome
Customer Account Password Reset
Fun Fact - Transactional emails are the highest opened emails you can send your customers!