The safety car is deployed to control the field while marshals deal with an incident on track. Cars must follow in single file and may not overtake, meaning that lapped cars are out of order.
Just before the safety car ends, the race director may allow the lapped cars to un-lap themselves by passing the safety car and rejoining the back of the field.
[caption id="attachment_4354" align="aligncenter" width="1024"] nytimes.com[/caption]