A Creo View adapter is the code to convert a certain type of CAD or doc data to a format that Creo View and Windchill understand. There is an adapter for all the major CAD applications, Office, ECAD vendors, STEP, PDF, etc. Each type you want to publish needs its own adapter. From that, you can create any number of workers to scale. A worker is configured to use the specific adapter to publish data on command from the server.
Think of it like people in a company, Each department processes different tasks. The workers are like employees in that department. If more jobs are coming into a certain department, you can scale up the number of employees to meet the demand.