Something you did in the past (this life or past lives) determines your future.
If you do good, good will be done to you.
If you do bad, bad will be done to you.
If you abused your wealth in the past life, you'll be pinching pennies in this life.
If you abandoned your family in the past life, you'll be all alone in this life.
Something like that.
What I've learnt in Reikongaku about karma so far is not so specific and easily explained.
This is an example of how I understand karma:
The source of karma is some event or a series of events in the past that left deep scars in your spiritual body.
These scars contained certain intense emotions such as fear, anger, misery, or despair.
When some event occurs later in your life, not necessarily similar in specific setting but more a setting that triggers similar emotions, these deeply embedded scars or karma take control and push you to react based on such past emotions, thus bring about the unexpected or undesirable consequences.
We can discuss and debate over this subject as long as we want, but more importantly, we need to work with our own karma. If it's a good one, we can utilize it. If it's an undesirable one, then we need to cure it before it takes control of our lives.
This is one of the main purposes and goals of spiritual training that we learn and practice.