Understanding audio output and resistor
Opamps generally don't like lots of capacitance on their outputs.  Why is a different question.
Long cables look like capacitors.  (2 conductors with insulator between)
R2 disconnects the amp from the cable to keep it from oscillating.


The tradeoff is that the device's output is weaker (higher impedance) than if the resistor wasn't there, which will make it interact more with external stuff including:


A long cable's capacitance to make a lowpass filter.
Another device with similar output to make a passive mixer, provided that they don't go into overcurrent mode because the output resistors aren't that big.




As for R1, I don't see that it does much of anything.  If this were an input, then it would keep it somewhat stable when nothing is plugged in, but for my designs I would consider it to be too weak to do much else.  The voltage divider effect is negligible because of the extreme ratio.