Re: Oscillator function
Microcontrollers and Microprocessors need oscillators to provide the clock signal required to make the synchronous internal digital circuits work.
When you learn about the basics of digital electronics, you see that flip flops, binary counters and registers need a clock to work. There are a lot of counters and registers inside a processor. Memories also need the reference of a clock signal.
In the PIC case, it can be an internal or external oscillator. There are several ways to make it work: relaxation oscillator (a RC network internal or external), a crystal/resonator oscillator that provide a more accurate clock.