There are two types of karma: Reciprocity (What comes around, goes around) and Religious. I believe in both. Karma actually means action or deed. It is good, it's bad, it is what it is. People often think it doesn't exist because life has so much suffering. Sometimes it looks like we don't have any justice for our misfortunes, but we do, eventually, in some way even if it isn't instantaneous. Our deeds always come back on us positively or negatively. Rapists, murders, these people may get off on a crime, but their lifestyle always comes back on them in some way. People who treat others badly eventually burn all their bridges and find themselves alone and miserable. People who treat others kindly will ultimately be surrounded by a supportive environment. It doesn't mean bad things won't happen.
Religious karma is much more involved and harder to grasp. It exists and is carried over between lifetimes. The goal is to balance the good and bad, to be neutral. Neutral karma stops samsara, the cycle of rebirth. Different cumulative actions lead to rebirth into six different realms (most often rebirth as a human again).
I understand if people don't believe in spiritual/religious karma, but day to day reciprocity is very real even if we aren't always seeing it. Experiment with it in your own life; be kind to everyone, even those who are unkind to you. The more you act kind towards others the more good things will come to you.