It's when you do something bad, stupid, malicious, cruel, take advantage of an innocent person, lie to them about yourself or your background, steal from them, be deceitful to others, and then what happens is that you will have the same happen to you but worse. It's THE AIR WE LIVE IN that gets back to you to teach you life's lessons that you shouldn't do that to anybody. Don't do stupid things to others and stupid things won't happen to you. You may think you're getting away with doing terrible things to others, but BAD KARMA always catches you and you will get it worse.
Good karma comes back to you threefold. Bad karma back to you tenfold. Think of this...if doing the righteous thing was easy, everyone would be that way. But it isn't easy & sometimes you'll stand alone. But I believe it's worth it. I sleep well and I ALWAYS treat others as I WISH they would treat me. Alas, not the easy path, and own up to your mistakes. There was only 1 perfect person & that was Jesus Christ. All humans no matter how righteous, are perfect. Thank god! How boring that would be.