I Would Like To Know What The Following FORTRAN 77 Code Does

In a .f file there is code that does this:

real Bob, avar
...
avar = Bob( 8.3 )
...

Bob appears to be a function but it is declared at the beginning of the file as a real.

Then there is a .d file that has a reference to Bob. Also I know avar holds a value that appears is interpolated, The problem is the interpolation is not always right.

Bob    John      35
-17.     -16.     -15.     -14.     -13.     -12.     -11.     -10.     -9.      -8.  
-7.      -6.      -5.      -4.      -3.      -2.      -1.       0.       1.       2.5  
 3.       4.       5.       6.5      7.       8.       9.       10.      11.      12.  
 13.      14.      15.      16.      17.  
 0.001    0.001    0.041    0.098    0.155    0.213    0.27     0.327    0.384    0.441   
 0.499    0.556    0.613    0.670    0.728    0.785    0.844    0.904    0.965    1.058   
 1.089    1.152    1.216    1.314    1.347    1.413    1.479    1.543    1.609    1.670   
 1.733    1.799    1.860    1.925    1.937 

This is all legacy code. I did not write it. I am trying to fix it. My question consists of the following:

What is this doing and how? What is a .d file?

I think it is trying to interpolate but I did not think you could do this (in this way) with FORTRAN 77.

Sorry for the confusion. The answer is the system is using a proprietary macro c to FORTRAN program that does interpolation. This happens in the make file. I found by looking at some obscure documentation . Thanks everyone for their input. Sorry again for the terseness of it. I was not trying to be difficult. It was confusing to me with what I saw as well. It is sometimes difficult working with 30 year old legacy code bought from a different company. I am new to FORTRAN so I thought I was not seeing something I should have been seeing like a language feature I was unfamiliar with. I feel foolish. It did lead me to dig deeper. Lesson learned.