A varistor is an electronic component that protects circuits from voltage spikes. It is made of materials that change their resistance based on the voltage applied. When the voltage exceeds a certain level, the varistor conducts electricity, diverting excess current away from sensitive components. Varistors are commonly used in devices like surge protectors and power strips to prevent damage from lightning strikes or power surges. They help ensure the longevity and reliability of electronic equipment by absorbing sudden increases in voltage, thus maintaining a stable operating environment.