The third renunciation thought is karma. Karma means that whatever we do comes back to us. When we love, love is returned to us. When we hate, we receive hate in return. That is the internal system of karma.

The process is the same in the external karmic system. When we plant rice seeds, we get rice. When we plant corn seeds, we get corn. Everything manifests according to its causes and conditions, which create inevitable results. This is not Buddhist dogma; it is a law of nature.

It is important to contemplate this so we know what is good and what is bad for ourselves and others.

Even an action that appears insignificant can have enormous consequences. Please be intelligent regarding karma, and very careful with your actions, because a tiny seed can bear a lot of fruit.

When we are fully aware of the inevitability of karma we empower ourselves. We all want happiness; no one wants suffering. Karma teaches us that positive actions bring positive results, which lead to happiness, and negative actions bring negative results, which lead to suffering.

A combination of positive and negative actions brings results that are a mixture of happiness and suffering.

What is the source of negative and positive actions? The great master Nagarjuna said that actions based on ignorance, attachment, and anger are negative, and actions based on wisdom, nonattachment, and loving-kindness are positive. This means that we create our own karma by what we think, say, and do—we are the agents and creators of our happiness and suffering.

Taking this to heart, we should work continuously to uproot all our negative mental tendencies and to cultivate and glorify the positive ones. We should continually manifest this in our words and actions for ourselves and others. This is the way to properly work with karma.

Source: Sherab, Khenchen. The Nature of Mind (pp. 120-121). Shambhala. Kindle Edition.

The Sutra on the Analysis of Actions states:
Parrot, the son of the brahmin householder To’uta, asked: “Oh, Gautama, what are the causes and conditions by which beings have long or short lives, good or poor health? Why are they handsome or ugly, powerful or weak, of noble or humble origins, wealthy or poor, intelligent or dull?”
The Buddha replied: “Son of a brahmin, sentient beings are the product of their deeds. The lot they enjoy is that of their deeds. The place they are born in is decided by their deeds. They are dependent on their deeds and are thus distinguished as sublime, base, or middling; as of high or low status; as good or bad. Beings have all sorts of deeds, all sorts of views, all sorts of experiences. Because of the negative actions associated with these, they take birth in the hells, or as hungry spirits, or as animals. Because of their positive actions, they will be born among gods and humans.”
(Taken from Dudjom Rinpoche, A Torch Lighting the Way to Freedom.)