It all comes down to beats really. On the top is the count of beats. On the bottom is the method for counting beats.
If you have 4/4, you have 4 beats. If you have 12/8 you have 12 beats. If you have 27/2 you have 27 beats. All of which are 'per measure'
The latter number is simply what constitutes as a 'beat length' in your piece.
In 4/4, your 4 beats are each, lets look at the bottom, quarter notes. So each measure or bar, should add up to 4 quarter notes. When writing within the bar, your writing is divided/added/multiplied so you have the total length within a bar of 'one whole note' or 'four quarter notes'.
In 12/8, a full bar would be equal to the duration of 12 eighth notes. Which ends up actually being the length of one and a half whole notes. so there ends up being a difference in the length of 'a bar' when using the grid system that Ableton Live uses. (go ahead and try it, set Live to 12/8, double click a new midi clip, and compare its length/structure to that of a 4/4
As for 27/2. You have a really screwed up length of 27 beats. There could be all kinds of weird swings/triplets/anything you imagine in there. But still, the total for one whole bar ends up being 27 half-notes (musically). It still just comes down to the type of symbol you feel most comfortable writing with. In this case, it would take 13 and a half whole notes, to even get one full bar/measure.