Active Job is a framework for declaring jobs and making them run on a variety of
queuing backends. These jobs can be everything from regularly scheduled
clean-ups, to billing charges, to mailings. Anything that can be chopped up into
small units of work and run in parallel, really.

It also serves as the backend for Action Mailer's #deliver_later functionality
that makes it easy to turn any mailing into a job for running later. That's one
of the most common jobs in a modern web application: sending emails outside of
the request-response cycle, so the user doesn't have to wait on it.

The main point is to ensure that all Rails apps will have a job infrastructure
in place, even if it's in the form of an "immediate runner". We can then have
framework features and other gems build on top of that, without having to worry
about API differences between Delayed Job and Resque. Picking your queuing
backend becomes more of an operational concern, then. And you'll be able to
switch between them without having to rewrite your jobs.

See also: https://rubyonrails.org/