A Process Definition is a definition of one or many Tabular Models. Each Tabular Model can be configured independently. A typical configuration is to specify the Process Mode and Process Type to be used for a Tabular Model.
A Process Definition can either be generated or defined manually.
Once the definition is ready, a Process Run can be executed, either directly or as a scheduled job. The run will start a job that runs an SSIS package that will execute processing for each Tabular Model or tables of a Tabular Model.