The start node is the entry point for a workflow job in Oozie, it indicates the first workflow node the workflow job must transition to.
When a workflow is started, it automatically transitions to the node specified in the start.
A workflow definition must have one start node.
Example here, so it will be clear for you to understand:
<workflow-app name="foo-wf" xmlns="uri:oozie:workflow:0.1">
The to attribute is the name of the first workflow node to execute.