Mild rewording of the question to follow.
You are asking about this line in the Darktable source code, and I applude that you are investigating the software at that level.
It's part of this section:
int main(int argc, char *arg)
if(argc < 2)
fprintf(stderr, "usage: %s input.pfm [-c a1 b1]\n", arg);
int wd, ht;
float *input = read_pfm(arg, &wd, &ht);
And the fragment broadly reads "if the program is started without enought arguments, then print an error message giving the normal usage.
I think you've slightly misread the command. It should be along the lines of
...and then an optional height and width.
I don't actually think this is a photography question (it's certainly far from a photography answer) so I'm voting to close on that basis...