Driving outside the groove means that the car isn't following the most efficient path around the track. That means that instead of being in the groove, the car is either too close to the outer wall of a turn or too close the apron, a term for where the flat infield meets a banked turn on a track.
As you can imagine, both situations can be disastrous for drivers -- for a variety of reasons. Being in the groove isn't only about maintaining speed and winning the race. Being in the groove also lessens the chance of an accident. Near the wall, there's always the chance of being pushed into the wall, or slamming straight into it when the car is coming out of the turn. Driving on the apron is no better. Staying in the groove and off the track's apron helps the driver maintain control. While everyday drivers may have driven onto the shoulder of a turn without disastrous results, at the speeds NASCAR race cars travel, the results can be catastrophic. One of the most famous examples of this is the on-track death of racing legend Dale Earnhardt at the 2001 Daytona 500. Earnhardt was the middle of three cars on the final lap of the race -- Daytona International Speedway has three grooves -- when his left rear fender was nudged by the car on the inside groove. That caused Earnhardt's car to hit the apron and lose control, sending it back up the banked track and into the wall, along with another car.
In terms of racing strategy, because the groove is the fastest and safest place to be, drivers try to get into it and keep other drivers out of it. To do so, drivers rely on teammates on the track for drafting and help with passing as well as other crew members who act as race spotters to let them know what the other drivers are up to. Even when they're in the groove, NASCAR drivers must constantly monitor shifting racing conditions, including the ever-changing positions of the competition, to maximize their advantage.
Driver positions aren't the only constantly changing factor during a race. Read the next page to find out how the groove can change during a race, too.