Start Flow is a step which allows you to direct your contacts to another Flow in your Flow Builder. The step has a connection dot like Action or Condition steps - an admin can use it to choose the next step which will fire after a contact finishes the flow selected in Start Flow step.
How to create Start Flow step?
To create a Start Flow step click on a round button in upper-right corner of the screen or just double-click any empty space in Flow Builder. Then select a Start Flow from the drop-down list.
Click on the Start Flow step, then click Click to Select a Flow. Choose a flow from the list and click on Select This Flow button. The last thing is selecting a flow or action that will fire to a contact after the content from the Start Flow step ends. To do that, simply click on the step's connection dot and drag the line to the other step.
Flow connected through "Start Flow" block will send all the messages connected on "Continue automatically" line and then the initial Flow will be continued. Connected Flow will not wait for contact's input on messages with buttons or quick replies.