Heroku dynos are containers provided by Heroku that help run processes within an application (In this case the TheatreDigsBooker website).
Workers or Worker dynos, can be of any process type declared in your Procfile (a mechanism for declaring what commands are run by your application's containers). Worker dynos are typically used for background jobs, queueing systems, and timed jobs. You can have multiple kinds of worker dynos in your application. For example, one for urgent jobs and another for long-running jobs.
For TheatreDigsBooker these workers refresh the system and run tasks such as changing a booking status to completed once the check out date has passed, or processing a photo to add to a Hosts website. Little background tasks that ultimately keep the site running.
In a review app, worker tasks are sometimes needed to be turned on to properly QA something before pushing to production.
To turn on a worker;
1) Sign into Herku here: https://id.heroku.com/login
2) Click the pull request for the review app you would like the worker to be turned on in.
3) Here click the resources tab, then the under dynos, click the edit icon across from the worker dyno.
4) Now click the sliding tab as shown below and click confirm.
The Worker should now be turned on.
Here has a nice little gif and more information on what dynos and workers are: https://www.heroku.com/dynos