The R1 only purpose it at startup, when the circuit is powered-up. It help the crystal to enter oscillation. Usually, you can omit it, but sometimes, it can happen that you get problems, with some circuits, where the oscillator won't start properly. It will either take a long time to start to oscillate or even not start at all. This doesn't happen often, or for every circuit types, but it's good practice to put a resistor.
The resistor must be of high value. It is normal for it to be 10Meg. This is because the crystal itself have a very high impedance (away from it's resonating frequency). And if you put R too low, the circuit may not oscilate at the crystal frequency, but in plain free-running mode (a much higher, uncontroled frequency, unsuitable for the application).