Karma,the natural law of cause and effect,is the truth and the driving force behind rebirth.It can be literally translated as "action",and is both the power latent within actions,and the results our actions bring.
There are many kinds of karma:international karma,national karma,the karma of a city,and individual karma.All are intricately interrelated,and only understood in their full complexity by an enlightened being.
In simple terms,karma means that whatever we do,with our body,speech,or mind,will have a corresponding result.Each action,even the smallest, will bear its consequences.Buddha said:"Do not overlook negative actions merely because they are small;however small a spark may be,it can burn down a haystack as big as a mountain." Similarly he said:"Do not overlook tiny good actions,thinking they are of no benefit;even tiny drops of water in the end will fill a huge vessel."The results of our actions are often delayed,even into our future lifetimes;we cannot pin down one cause,because any event can be extremely complicated mixture of many karmas ripening together.
Just as the fact that everyone is relatively imperfect, but ultimately perfect,a buddha, so is karma a relative truth, and since it is relative it is changeable.If karma were ultimate truth,a buddha could never exist,because how could anyone become buddha?It would be impossible if karma were irrevocable,ultimate truth.