Summary: PNR numbers are used by the airlines use to access your booking info.
A Passenger Name Record (PNR) is a unique number which the airlines use to access your booking info.
Multiple PNRs You could have multiple PNR numbers if you have more than one flight or if there are multiple passengers in your booking.
Visa implications When you have multiple PNR numbers, you may need to leave the transit zone of the airport. You should find out if you need an airport transit visa because of this.
Refund implications Airline refunds are processed by PNR number which is why your refundable amount may be displayed per PNR.
PNR location You can find your PNR number(s) on the top left of each page of your e-ticket as per the screenshot below:
What if I can't find a PNR on my e-ticket? Don't worry, it means that your booking has been made but the carrier hasn't issued your PNR number yet. You should receive a new e-ticket with the PNR number within 24 hours.