karma. A term many of us hear everyday.
karma (deva: कर्म, from sanskrit root कृ "to do"/"to make") is a very heavily loaded word. It always has been. One can see by the myriad of varied uses it has in sanskrit how loaded the word is. kR, as the "to do" verbs in many languages are wont to do (seriously, no pun intended), you have tons and tonnes of related terms and phrases, idiomatic usages, and loaded meanings associated with the verb. Let's take things a little bit at a time, shall we?
Action. If we take kR as "to do" or "to act," we get "action." Simple enough, at least until we consider the repercussions of an action. When we act, there is a result from that action. We call that a reaction. The idea of karma is that of action and reaction. It's not enough to think of what we do; we must also consider what kinds of effects that has and how those effects propagate. Newton's laws and forces do well to illustrate karma, really.
When we act, the effects ripple outwards. We affect other things and people when we do something, anything, regardless of how small the action. I won't get into "butterflies flapping their wings" here, but it's true. Let's take this into a less concrete layer here. When one acts towards another it can be considered either good, bad, or neutral. After all, when we think of karma, we think of a sort of "point system" do we not? Thus, we see a reaction. Usually, when people are acted upon positively, they respond by feeling positive. Sometimes, they also do positive things back, whether directly to the actor or to others. When people are acted upon negatively, they respond in kind. When people are acted upon by a neutral act, they usually think it's weird, ignore it, or take it as good/bad and act in kind. Sometimes they do all of the above. Now, when we think in the realms of human experience and action, there's one important thing here to consider: the reception of the action. If people take things to be good or bad, they act in a way that propagates that feeling. On occasion, however, we can see people who act differently.
Ever see the annoying optimist? That really annoying person who takes good news from bad news, or tries to always see the bright side of things? How about the nay-sayer? How about those people who really get overly dramatic over nothing? It's useful to try to take a bad situation and make it better, but when someone tries to do things for us, or doesn't do it in an incredibly sensitive way, we easily get annoyed. At the same time, when good things happen, some people just don't want to take it. Usually, it's a way to preemptively divert disappointment that may come when a good thing turns out to not be so good. At any rate, we can see this as a way to change the karmic effects of an action.
People sometimes make something out of nothing, too. Usually, this is because something passive strikes a nerve of a person and they end up taking it in a negative way. Since you can't make something from "nothing," per se, you have to assume there's some sort of action that doesn't have any good or bad effects in and of itself. That's what neutral karma is. And, there are those that try to appreciate the subtleties of life. They look at things that affect them neutrally and change them to have positive effects. Now that I think about it, being genuinely happy for others sort of falls into this category as well.
If you can change your reception of an action, you can change how it affects you, how you respond, and how that response can affect others (depending on their reception). Take that piece of advice any way you will.