Karma is simply cause and effect - for every thought, word and action there is a corresponding reaction. Living creates karma. Over a lifetime and many lifetimes our karma accumulates.
If karma is being generated from a less enlightened way of being then what is accumulated will reflect this and individuals may find themselves attracting negative circumstances around health, work or relationships. A karmic clearing may heal any such situation - erasing this negative impact and opening the way for a more positive life experience.