This video explains Clocking MCU by Internal PLL (Running MCU at Full Speed)
STM32F4xx based MCU is used in this video and it clearly explains all the PLL aspects of the Microcontroller.
PLL is a clock generation engine in the MCU which is used to generate the clock speed which is much higher than the internal HSI or external clock . In Mcu most of the peripheral like USB,Ethernet PHY can not work if you clock the MCU by low speed HSI or HSE. so , in those cases you can take the help of PLL.
Remember that in MCU PLL is not enabled by default, it must be configured before you use it. This video explains how you can configure the PLL to generate high frequency clocks.