The framerate (fps) measures how many frames are rendered by Millumin per seconds. Usually 50fps or 60fps.
The framerate is managed by OSX. Indeed, it connects the hardware and synchronize your monitors all together : if they are at 50hz, the final framerate will be 50fps. If they are at 60hz, the final framerate will be 60fps.
And Millumin is synchronized to this final framerate.
If your graphic card cannot render fast enough, the framerate can be lower : check it by clicking on "Output" then "Show FPS" in the menubar (and if the framerate is lower, click on the "Optimize" button).
If you run movies at 24fps, 25fps or 30fps, no problem : Millumin will render all the frames.
However, if you run a movie at 50fps, it is better if your monitor work at the same refresh rate (50hz). So movie frames are synchronous to framerate (and all movie frames are rendered).
For info, the "global framerate" in Millumin V1 Preferences, is an old option, and is not relevant any more (see the "i" icon for info).
In brief : it simply served to skip frames during rendering (for example, rendering at 30fps, so skipping 1 frame out of 2). But this is a bad practice, and it does not allow to get better performances any more.
That is why it has been removed. To remove confusion and mistakes.