PLL is the phase-locked loop electronic circuit, usually a VCO (Voltage Controlled Oscillator), that generates the various clock signals for components like the CPU, PCI Express bus, Memory bus, PCI bus, etc. They are usually configured as frequency multipliers, dividers, demodulators, tracking generators or clock recovery circuits.
Consumer PCs, including notebooks, don't have a full-featured (i.e. unlocked) BIOS that would allow you to change settings like Internal PLL Overvoltage or CPU PLL voltage or PCH PLL voltage. None of these companies want to support the problems that would arise if settings like these were exposed in the BIOS and some noob changed the settings to the point where the PC won't run properly.