A Phase-Locked Loop (PLL) is an automated closed-loop electronic feedback control system that continuously aligns the output frequency and phase of a Voltage-Controlled Oscillator (VCO) with an incoming reference signal. A phase detector measures phase differences between reference and feedback signals, generating an error voltage filtered by a low-pass loop filter. The resulting filtered control voltage tunes the VCO until the phase error vanishes, achieving synchronized frequency and phase lock.