Hi there
PICs can handle atmost one interrupt (if priority is not set) at a time. Thus writing interrupt in front of any function makes it an ISR (interrupt service routine). As in this case, PIE1.RCIE=1; statement is written to enable the USART Rx interrupt; so any interrupt request by Rx's flag will make PIC start executing the void interrupt();
Also this same interrupt function can be used to see if any other interrupt request has been made or not. Refer its datasheet for the same.
Hope this clears your doubt 