There is a commonly held notion (popularized, I think by New-Age ramblings) that karma is some kind of force in the universe that metes out absolute punishments and rewards according to the relative actions of individuals.
If this is your understanding of that term, it is mistaken.
Ok, so what then is your understanding?
The short answer is that
karma is how the unenlightened mind experiences the interdependence of phenomena.
A buddha experiences the interdependence of phenomena without experiencing karma and rebirth.
Karma is a mental experience.
It is the experience of the actions, the experience of the cause of actions and the experience of the result of actions
of one's own body, speech and mind, somewhat deluded by clinging and attachment.
The universe doesn't care what you do.
Ethical concerns of good and bad are entirely constructs of the human imagination.
The positive or negative actions of your body, speech, and mind only bear fruit
because of, and to the degree that you grasp at appearances as inherently existent.
It is one's own mind that produces, and experiences rebirth under various circumstances.
So, when it is said that a person who is greedy in this life will be reborn impoverished in the next
it doesn't mean they will be poor. They may be reborn in a wealthy, royal family
but in their mind, they will never be satisfied. They will never have enough.
That's because, in the mind, they have planted the seeds of dissatisfaction.
Always wanting more and more of everything, naturally one can never be satisfied
so, the karma of this is that the person will, in the future,
always feel as though they don't have enough,
even if in fact they do.
Or, a person might be reborn from this as a hungry ghost.
What does that mean? Hungry ghosts are always hungry and thirsty
They are so messed up, that even if you give them water and food,
they perceive it as vile pus and filth.
Are they real?
Real as compared to what...compared to us?
real compared to us?
Whether hungry ghost really exist is not the point.
Believing in them or not is of little consequence.
This is a description of a state of mind. even if it is delusional on the part of those who experience it,
it is still as real an experience as our own human realm.
Conversely, if you are generous, you feel rich even if you own very little.
But what causes this isn't some grand scale of judgement in the cosmos.
It's all within one's own mind.
The other realms are no more real than this one
but look at how real this one seems.
As a result, our experience of karma seems just as real.