What does N/F mean on the resistors?

Not fitted. In this case, specifically, the designer wanted to be able to choose whether IN+_2 and IN-_2 should each be connected to either IIN_2A, IIN_2B, or IIN_2C. That's why they used a combination of zero-ohm resistors (which are like wires) and N/F parts.

By selectively mounting zero-ohm resistors to different pads, it allows the designer to reuse the board for different scenarios, or with unknown future configurations.

In the default configuration specified by this schematic, IIN+_2 is directly attached to IIN_2A, and IIN-_2 is directly attached to IIN_2C and IIN_2B.

Because R64 is connected to ground, I assume there was an optional voltage-divider (or digital pull-down) resistor on the design, too.

Not fitted. Used when you want a footprint on a PCB, but don't necessarily want to install a component there.