A Phase-Locked Loop (PLL) is an electronic circuit used to synchronize an output signal with a reference signal in terms of frequency and phase. It consists of three main components: a phase detector, a low-pass filter, and a voltage-controlled oscillator. The phase detector compares the phase of the input signal with the output signal, generating an error signal that is filtered and used to adjust the oscillator. PLLs are widely used in various applications, including radio communications, clock generation in digital circuits, and frequency synthesis. They help maintain stable frequencies and improve signal quality, making them essential in devices like radios, televisions, and computers.