A phase-locked loop (PLL) is an electronic circuit with an oscillator in which the frequency is constantly adjusted to match in phase (and thus lock on) the frequency of an input signal. PLLs are often used to stabilize a particular communications channel (keeping it set to a particular frequency). A PLL can be used to generate a signal, modulate or demodulate a signal (as in FM), reconstitute a signal with less noise, or multiply or divide a frequency. PLLs are frequently used in wireless communication, particularly where signals are carried using frequency modulation (FM) or phase modulation (PM). PLLs generally take some (small) amount of time to adjust to changes in the input signal so if the circuit is designed accordingly they can also act as a sort of buffer or stabilizer for an input signal with a lot of irregularities. Think of it as a sort of frequency flywheel.