Error amplifier in dc dc converter chip
The error amp is connected to the secondary labeled "NC" in this diagram. From there the signal gets rectified (by D3 and D2) and filtered (That chunk of Rs and Cs) before going through a voltage divider (R3 and R4) to pin 2. R2 probably provides some sort of DC bias to the feedback signal, probably to prevent it from destabilizing during start up or heavy load transients. 

As a side note: That datasheet is oddly incomplete for TI. Usually there is much more information about the application and the per-pin requirements and recommendations. Possibly there is an App note you could look at for this part that could explain some of the choices they made in this circuit.