How do I interpret these values in NASA's C-MAPSS dataset files?
difference between nominal and fault file
  


It is found inside the .tar.gz file. From the README file contained in the page you linked:


  the Simulation_Info.txt contains a summary of the number of flights,
  the fault type that was simulated, if it is nominal or not, and if not
  what flight and time sample the fault was introduced.





  
  There are 27 parameteres used, among these which parameters will affect fan, HPC, HPT, LPC, LPT
  


I assume you are asking what are those 27 parameters.
From the README file that is found in the page you linked:






  
  what is the threshold value for these parameters or when do we say that the parameter has reached its max value and above which it would lead to fault condition.
  


A detailed answer is out of scope for this site, it heavily depends on the failure simulated. The documentation of the various tests should have enough details to understand.

Additionally, hard absolute values are definitely of no use here, different engines will have different characteristics: the numbers valid for the engine simulated here will be applicable to basically no real engine.