A Phase-Locked Loop (PLL) is an electronic circuit used to synchronize an output signal's phase and frequency with a reference signal. PLLs are vital in communication systems for generating stable frequency signals, demodulating signals, and synthesizing frequencies. They consist of a phase detector, a low-pass filter, and a voltage-controlled oscillator (VCO). By continuously comparing the phase of the output signal with the reference, PLLs adjust the VCO to maintain phase and frequency alignment. This ensures accurate signal transmission and reception, crucial for applications like radio, television, and telecommunications.