The flags
Green starts the race and every restart. Yellow means a caution: cars slow, hold position behind a pace car and close up into a queue. Red stops the race entirely, usually for weather or for a crash that needs the track cleared properly. White signals the final lap, and the chequered flag ends the race. A black flag orders a car to pit, either for a penalty or because officials judge it unsafe to continue.
Because the flags are absolute, they matter more than any strategy plan. A caution thrown one lap before a scheduled stop can hand one driver a free pit visit and cost another half a minute, which is why teams listen to spotters and race control as closely as they watch the track.
Stages and restarts
A race is divided into stages. Each stage ends at a set lap, at which point a caution comes out and the leading finishers of that stage earn additional points. The field then pits, lines up and restarts. The effect is a race with several natural sprint finishes rather than one long run to a single flag.
Restarts are double-file, so the cars form two lanes. Choosing a lane is a real decision: one groove usually offers more grip and the other more room. Drivers who pick well can gain several positions in a single lap, and drivers who pick badly can lose the same in the same distance.
Pit road
Pit road has its own rule book: a speed limit, a defined box, a limited number of crew allowed over the wall and rules on equipment leaving the box. Most penalties in a race are committed here — speeding on entry, a tyre rolling loose, a crewman over the wall too early — and the standard punishment is a pass-through or a restart at the back of the field.
That makes pit road the most concentrated risk in the sport. A stop gains a few seconds of track position at best, but a penalty there can cost a lap, which is why crews rehearse the sequence relentlessly.