Oh dear .. what a load of rubbish on this thread.
Certainly we must dismiss the various pseudo-scientific babble and childish schema for "justice", punishment or reward and winning lotteries in this life or another.
The idea is not properly defined, there are multiple different definitions. The meaning it has is poetic, mythological, metaphysical (ie no longer considered even as philosophy). An idea that can be meditated on and understood personally but really not really communicated except through leveraging empathy. It offers no logic that can be understood to be true or false.
The
Wikipedia entry on Karma is pretty good. It includes this poem, which seems to capture the essence ...
Now as a man is like this or like that,
according as he acts and according as he behaves, so will he be;
a man of good acts will become good, a man of bad acts, bad;
he becomes pure by pure deeds, bad by bad deeds;
And here they say that a person consists of desires,
and as is his desire, so is his will;
and as is his will, so is his deed;
and whatever deed he does, that he will reap.
— Brihadaranyaka Upanishad, 7th Century BCE
It is not an answer to anything, not even attempting to define what "pure", "good", "bad" are, but only somewhere to start your own introspection.