At the moment, "email addresses are automatically added to your Contacts list each time you use the Reply, Reply to all, or Forward functions to send messages to addresses not previously stored in your Contacts list," according to Gmail's help center. Gmail also adds the email addresses used when you compose a message.
The new option will allow you to disable this feature, but Google's description is strange: "Create contacts (sets whether sending or receiving a message can create a new contact)." Right now, Gmail doesn't automatically create contacts when you receive new messages. You need to reply to the messages first.
Here's the video (you can fast forward to 1:14 min).
{ Thanks, François. }