"Riding shotgun" means sitting in the front passenger seat next to the driver when you're in a car. It's a term that comes from the old days when people would ride in stagecoaches or wagons. The person who sat next to the driver was called the "shotgun messenger" because they would carry a shotgun to protect the passengers from robbers and other dangers on the road.
Nowadays, riding shotgun in a car isn't about carrying a weapon, but it's still a cool way to ride. Kids who want to ride shotgun need to be old enough to sit in the front seat and follow the rules of the car, like wearing seatbelts and not distracting the driver. But riding shotgun is a great way to have a better view of the road and to talk to the driver, making car rides more fun!