A sub workflow is a branch of a «parent workflow». At the point where a sub workflow is inserted, the parent workflow is paused until the sub workflow has been completed. Sub workflows are passive connectors.
To set up a sub workflow, select the option 
when creating a new connector:
After clicking on 
you can define which parameters this sub workflow should receive. Give the variable a name and define the type:
The same can now be defined for the outgoing parameters:
When creating a new workflow, you can now select the Sub Workflow Connector and set it up.