Bad Karma is when you have something bad happen to you as a result of doing bad things yourself.
For example you bully someone and call them names, etc and then eventually you lose a loved one or you get into an accident yourself.
That would be bad Karma.
I've personally witnessed bad Karma.
I had a bully in school who would always bully me and some other kids and eventually bad karma caught up with him.
As soon as he got his Drivers License he got into a bad car wreck with a semi truck and almost died.
He was very lucky to be alive and after the car wreck he apologized about his wrong doings and said that him being in the car wreck changed him.
So basically you get what comes to you for doing bad things.
There's also good Karma.
If you do good things then good things will come to you.
But if you do bad things then bad things will come your way.