The Validate function checks all the nodes in the master flow to determine if they are valid. If an error is found on any of the nodes, an error is displayed, denoting the data flow node(s) where the error is located.
To check that the master flow is valid, click Validate from the ribbon (red highlight below):
If the flow is valid, you will see the green 'success' message.
If there are any invalid nodes, you will see the red error message. You'll also see all invalid nodes listed in the Errors panel: