A reserve driver is a Formula One racing driver that is ready to race, in the event that a team's primary drivers cannot participate in a Grand Prix, most commonly due to illness. They are often at the circuit during a race weekend. They are specially trained to jump into the car at short notice and participate in the race if needed.
It is common to associate a reserve driver with a test driver since they both are not primary drivers for a team and serve in a support role. However, they have slightly different tasks on a Formula One team. A test driver, or test and development driver, usually assists a team by racing on a simulator to gather the crucially important data points that teams need when developing their cars.