Hi Isamu. Welcome to Racedepartment.
Stop n Go is a penalty you incur if you cut a chicane mostly. You have to make a pitstop and stop at your pit box.
Yellow flags will be shown when there is an incident on the track, maybe an accident or someone went of the track. Strangely enough the guys says no overtaking but in offline mode the AI will zoom pass me lol. When I see yellow flags, I am cautious and look ahead for any mayhem to avoid.