The idea of Karma is that what you do and the way you behave affects what will happen to you in the future. Its an idea that some people like to live their life by.
So basically if you do good things and good deeds then you will have good karma, and good things should happen to you in the future according to this idea.
Likewise if you do bad things then bad things will happen to you, as you will have bad karma.
That's why if someone does something bad or wrong you might hear someone say 'there's some bad karma coming his way' or 'that's bad karma.' It means they think or hope something bad will happen to that person to make it fair, as they did something bad.
Of course it doesn't always the universe doesn't always work that way, but it would be nice if it did.