Suggested readings: Transactional Journeys, Canvas and Navigation, On API Call Starter
Wait elements let you pause users between two messages in a transactional journey. With them, a single API call can deliver a whole sequence, such as an order confirmation now and a preparation update two hours later, without your service having to call the journey again.

You can choose one of the wait element types based on your use case:
Transactional Journeys support these two wait types only. Wait for Attribute, Wait for Event and Wait Until Dynamic Time are available in promotional journeys only.