People often struggle to understand karma. Karma is simply the law of cause and effect, and in our lives this law of cause and effect depends on our intention. To understand, notice how the motivation or intention preceding an action determines the future karmic result of that action. If an act is motivated by true kindness, it will necessarily bring a positive result, and if an act is motivated by aggression or greed, over time it will eventually bring an unpleasant result. But because karmic results do not always bear fruit immediately, it is sometimes difficult to observe this process.

Speech is one area in which karma can be seen in an easy and direct way.
Karma, Intention & Wise Speech — by Jack Kornfield