That is not a simple question, it is a mind reading exercise. You have not said where that code comes from or what any of the variables are or how it relates to the “reson” filter.
Surprisingly, my crystal ball is working quite well today 
and I see that you are referring to this code:
;control f "medium frequency" int "[Hz]" 1000 100 4000
;control fl "lfo frequency" real "[Hz]" 1 0.1 5
(reson s (sum f (mult f 0.5 (lfo fl)))(/ f 2.0) 2)
From this post: few basic problems - #2 by steve
In future, please do not expect my crystal ball to work. Please give the full context of your question (as above) so that other people on the forum can understand what you are asking.
“f” corresponds to the “centre”.
(/ f 2.0) is half of f.
The “2” at the end specifies the overall RMS value of the amplitude response is 1.0; thus filtered white noise would retain the same power.
http://www.cs.cmu.edu/~rbd/doc/nyquist/part8.html#index426