NASCAR Flags - NASCAR.com

Green
The track is clear and cars may proceed at speed. This flag is used to signal the beginning of the race and any restarts.

Yellow (caution)
The track is not clear, slow down and hold your position behind the pace car. This flag is used to signal an accident, debris caused by contract or mechanical failure, or weather-related issues. NASCAR rules allow cars to bunch up behind the leader. In most cases, lead-lap cars restart in the outside lane, while any lapped cars restart to the inside. In addition, a yellow flag during a practice session means cars should go to pits immediately.