Spinning reserves are backup power resources that can be quickly activated to maintain the stability of the electrical grid. These reserves typically come from power plants that are already online and generating electricity but are not operating at full capacity. When demand suddenly increases or if another power source fails, these plants can ramp up their output within minutes. This capability is crucial for managing fluctuations in electricity demand and ensuring a reliable supply. Grid operators monitor the system closely to ensure that enough spinning reserves are available, helping to prevent blackouts and maintain a steady flow of electricity to consumers.