The loop code is executed once for each frame being processed.

Typically, an effect's loop code would appear quite similar to the following:

&rfx_sendmsg (
  qq{
    (let* ((input-file (string-append "$curtmpdir" DIR-SEPARATOR "$in"))
           (image (car (gimp-file-load RUN-NONINTERACTIVE input-file input-file)))
           (layer (car (gimp-image-get-active-layer image))) )
      ;
      ; Your script code goes here, modifying the 'image'
      ;
      (rfx-save-frame image "$out") )
    }
  );