In Data Exchange Framework, models are used to represent a part of a sync process that performs a specific purpose. For example, the model for a pipeline represents the order of steps that synchronize data (such as sync data from an ERP to Sitecore xDB). The model for a pipeline step represents a specific function in a pipeline (such as read data or map data).
The model represents what can be done. The implementation of how that work is done is handled by a processor.
A part of the configuration process is associating a model with a processor. This is usually pre-configured in the Sitecore template that represents the model (using standard values). But it is possible to change the processor associated with a model at any time.