Workers are used to process build and deployment jobs.
Global workers are available for building at rocketgit.com, so just configure a build webhook.
Key - it is used to authenticate the connection from worker.
Cost - it is used to decide on which worker to send a job; bigger = less chances to choose it.
Max parallel workers: you can limit how many simultaneously jobs are sent to the worker.

To learn how to configure a build machine, go here. The worker will connect automatically and will be ready to receive build jobs.