The Buddha had some very specific, original teachings on karma that have much relevance to the meditation.

"We don’t often think of the teachings on karma as having much relevance to the meditation. Sometimes we’re even taught that karma was one of those weird pieces of cultural baggage that somehow got smuggled into Buddhism from its cultural background. But that’s not the case at all. The Buddha had some very specific teachings on karma that had nothing to do with what anybody else was teaching at the time, and they’re immediately relevant to why and how we’re meditating.

The “why” has to do with the point I just raised. Given that karma is intention, and intention is the huge shaping force in your life, you want some control over it. If you make up your mind to do something that you know is good, you want to be able to stick with that intention. And where does intention happen? Right in the present moment. Where does it get changed? In the present moment. This is why we focus on the present moment, so that we can see the process of intention in action as it happens and can have a say in where that intention is going to go. The more solidly you can stay in the present moment — the more steadily you can maintain your balance here — the more you’ll be able to see, and the more conscious say you’ll have in the direction those intentions are going to take you. That’s the “why.”

As for the “how,” you’ll notice as things come up in the meditation that the vagrant intentions have very little to do with anything you were consciously thinking about as you sat down to meditate, when you made your intention to stay with the breath. And yet suddenly they appear. This relates to the Buddha’s teachings on how your present experience is made up of three things: the results of past intentions, the actual process of intention in the present moment, and immediate results of that present intention. Certain thoughts are going to come up as a result of past intentions, and they don’t necessarily have much meaning. They just happen to pop up and they can be pretty random.

Sometimes we look for inspiration or signs of some special knowledge as we meditate. That can happen, but it’s also mixed up with a lot of really random stuff. It’s like looking for meaning in your dreams: Some dreams are portentous, some are pretentious, and most are totally random. You can’t take them as a dependable guide. In the same way, you can’t necessarily take what pops into your mind in the present moment as a guide either, no matter how still or luminous your mind may be, for a lot of what pops up is simply the result of random past intentions. But what you can do — by staying solidly in the present moment and solidly with your intention to stay with the breath — is, over time, to put yourself in a better position to evaluate what comes into the mind. If a thought of greed, anger, or delusion comes in, you’ll be able to sense it and to see what it does because you’re more sensitive to what’s going on here."

~ Thanissaro Bhikkhu "Sticking with an Intention" (Meditations3)