In Hinduism and Buddhism it is a very serious thing. Since I don't belong to either religion I don't understand it well, but it has something to do with how good or bad actions in this life can affect your destiny in your next reincarnation.
In the United States the word is used in a light-hearted way to mean "this is the kind of good action which deserves to be rewarded with good luck." For example, "always feed the seagulls, it's good karma."
It can also mean "I have a good feeling that good things are about to happen." For example, "I love hiking up Buck's Hill, that place just seems to be full of good karma."