in simple terms, what does karma mean? it means that whatever we do, with our body, speech, or mind, will have a corresponding result. each action, even the smallest, is pregnant with its consequences. it is said by the masters that even a little poison can cause death, and even a tiny seed can become a huge tree. and as 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." karma does not decay like external things, or ever become inoperative. it cannot be destroyed by time, fire, or water. its power will never disappear, until it is ripened.

sogyal rinpoche

read this