Waiting versus carrying on
On a phone call, you wait for the other person to respond before the conversation moves on. With email, you can do other things and then read the reply when it arrives. Software uses the same distinction.
During checkout, a shop may wait for payment approval. It can then add a receipt task to a message queue and confirm the order without waiting for the email to be sent. Another part of the system sends it later.
Why it matters
Asynchronous work can keep slow tasks from holding up the user, especially during busy periods. But the product must track tasks that are still pending or have failed. Asynchronous does not mean instant; it means other work can continue while the task finishes.