The PIC knows that an interrupt has occurred when

1. the event is triggered by hardware and sets a flag inside the PIC silicon
2. If you have the interrupts enabled and have a service routine for it, the moment the event occurs, the interrupt service routine (ISR) will service it and return back to what it was doing at that point in time.
3. The RB0 pin is also called INTE pin and is the one that triggers the event in your code.
4. This is the line that enables the INTE and GIE bits in the interrupt register telling the PIC whether to handle the interrupt in code when it happens
Code:
INTCON = $90 ' Enable INTE interrupt