A frame is a time slice in flash player. So if your fps is set to 12 fps, the script in that frame will be given attention (executed) for 1/12th of a second.
addFrameScript
is hardly documented, perhaps to discourage it's use by developers. So basically, in code you initialize the timer class & manually manage your time slices.
Frames are again represented as a zero indexed array, with collection of executable called the frame script. So when you add the framescript yourself, you are simply doing what constructor of the Frame Class (internally) might do if you put it on the as3 panel of the frame in IDE.
In short, addFrameScript(0, frame1);
is somthing in parallel to frameScripts[0] = frame1;
where frameScripts could be an internal array.