DNS propagation is the delay between adding or changing a DNS record and every resolver on the internet picking up the new value. It is not one global switch flipping at once; it is thousands of caches around the world each expiring on their own schedule.
The wait is usually short for a new record, minutes to a couple of hours, but genuinely slow when you change nameservers instead, since that hands the whole domain to a different DNS provider.
For what actually determines the wait and how to tell it apart from a record that is simply wrong, see connect a domain you already own.