What does the RNF pin do on an H-bridge?
If you look at the block diagram, it's apparent that those pins are where the H-bridge return current comes out. The reason they're labeled RNF and not GND, is that often times you will see a low value (e.g., under an ohm) resistor inserted in that return path. The resistor provides a voltage drop proportional to the current the bridge is passing. If you measure the voltage and you know the resistance, then you can compute the current via good old Ohms law.

You can connect these straight to ground if you like, and ignore the current, but knowing the current is really useful. It gives you an idea of the kind of load you have on the system. For instance, if you're driving a motor, then the difference between a happily spinning motor and one stalled out (and therefore probably overheating) is that the latter will pull a lot more current.