Most people have heard of the word karma, but most of those people do not know what it means.
"Karma" itself simply means "action" and of course every action follows a reaction and so on.
So overall the concept is not that complicated and mystical, except the part where it comes to reincarnation.
You do something and you get a reaction, very simple concept. The mystical part is that karma can also be carried over to your next life, which most people cannot accept or cannot understand, since they do not believe in reincarnation. But most cases of daily life do not need that explanation, it is only the bigger picture that involves reincarnation, most of the things in life do involve karma that is done and received in the current life. Most people dismiss the whole concept of karma, just because reincarnation cannot be proven and then throw away all the rest that happens in the current life, which can be proven.
Karma is also not so much a concept of a god sitting above and judging everything you do and then giving you your "karma", karma itself is very neutral, you just receive what you sowed.
You can sow good actions and receive good reactions and you can sow bad actions and receive bad reactions, but on the highest level there is no distinction between those, an action is an action, it is just that some are wiser than others.
Another mistake is that many people do is that they contribute everything that happens to karma, but this can hopefully be disproven by the fact that it needs actions to have reactions, not everything can be a reaction.
So it is possible that bad things happen to good people, or the other way around, in fact this often happens. Each of those actions will have a reaction later as well, but not in form of a god that judges and deals out punishments, it is more like a system of laws of nature, god was probably intelligent enough to automate that system, otherwise he would have to manually make decissions all the time.
Ultimatively karma is also defined as intention, so the intention of an action matters as well. Since on the highest level karma is defined as doing something and expecting a material gain in return. The main metaphysical aspect of the karma philosophy is, that karma ties one into the material world, since you expect a material gain, so you have to reincarnate to receive your karma that you sowed before in case you did not finish it in your current life. As a result of this there are many spiritual paths that offer techniques to getting rid of your karma to ultimatively receive salvation, which I will go into detail later, this should just cover the basics.