A phase-locked loop (PLL) is an electronic circuit that compares the phase of an input reference signal with the phase of a signal derived from its output oscillator. It adjusts the oscillator frequency to keep the input and output phases matched. A PLL consists of a phase detector, low-pass filter, and voltage-controlled oscillator (VCO). It is used for synchronization, frequency synthesis, and demodulation in applications like wireless communications, radio transmitters, and signal recovery in noise.