I don’t know about other programming languages, but if you are using Python or Django, you must have heard about Celery quite a few times, and if not, you better look into it. As stated on the project Celery website:
Celery is an asynchronous task queue/job queue based on distributed message passing. It is focused on real-time operation, but supports scheduling as well.
In case of a web service (most common use-case), asynchronous task queues are utilities to push (time-consuming) tasks in background while timely sending back the response for a user request. These delegated tasks can be anything from sending few notifications, dispatching emails, update system logs, or update internal ERP. Having the aforementioned tasks in line with the request processing, can delay the response back to the user to a large extent.