Start data is data that must be collected when the workflow is started.  When a user starts the workflow, they will be presented with a form requesting this information. 

Note: Start Data will only appear if the workflow is set to start manually, if the workflow starts automatically, the start data fields will not appear and if default values are set, they will be used for the variables.

Start data is configured by creating workflow variables and setting the Show on start form option to 'True' for the variable to be shown.

For more information please refer to the Workflow variables.