Karma is one thing a lot of people believe in. For most, it guides their actions. They believe that if they do something good, they will be paid back with good, and so it encourages them to do more good.
On the other hand, people believe that if someone does bad to them, something bad would happen to that person, no matter how long it will take. So in a way, this is like a check to people's actions because of the anticipation of consequences.
On several occasions, I've heard people say that someone going through something bad probably did something in the past to deserve what they are going through. Also, people believe that the good happening to them at the present moment is because of something good they did in the past.
Personally, I don't think karma really exists. There are people who have done some bad things in the past, yet good things continue to happen to them in the present. Life keeps going well for them.
Let's take corrupt leaders for example. A lot of them stole money meant to better the lives of the masses. A lot of them did very terrible things. Yet, they continue to walk free, they enjoy life and enjoy the money. Even when they're out of office, they get good appointments and jobs, their kids become well established.
This tells you that karma does not really exist.
From another angle, there are those who have mostly done good in their life, yet, terrible things continue to happen to them. Sometimes, such people are left wondering what they ever did to deserve such terrible things. That's just how life is.
Let's not confuse karma with immediate results for actions or inactions. For example, if you don't study, you're definitely going to fail exams. If you don't save, you'll go broke. If you don't plan ahead, you'll get stranded. This is not karma. It is just a basic life principle that actions and inactions have results.