Despite the title above, it is not possible to read a little about karma and understand it correctly (unless you've studied it at length in past lives and that prior knowledge gets triggered). Karma is the real law, as opposed to our earthly version, and if it takes a lifetime to understand our version, it would take proportionately longer to understand the real version.
A study of the principles of reincarnation generally brings with it an understanding of karma, and an immature understanding of karma is sometimes used to defend the stance that people are getting what they deserve, and there is no need to feel compassion for them nor to help them.
This is a temporary stage. A mature understanding of karma leads to great compassion, because one understands that each soul, while inherently pure and beautiful, is getting itself trapped in all types of suffering and addictions through spiritual ignorance. None of us are immune from it, especially if we could look back over our past lives, and none of us are in a position to judge anyone else. We are, however, in a position to understand, especially if we have been through something similar in one or more of our past lives.
Once this is clearly seen, then the response, to express this compassion, is two-fold:
1) service to mitigate or reduce suffering, and
2) removal of the root cause that leads people to create suffering for themselves, which is spiritual ignorance.
In cases where it is not possible to entirely eliminate suffering, sometimes if one looked deeply enough one would see that the more crucial and more lasting work of removing spiritual ignorance requires that it continue for a time.
The profound difference between compassion which is based on materialistic assumptions about life, and compassion based on a mature understanding of reincarnation and karma, is that the former believes that everything is wrong in the world and must be fixed. The latter, while recognizing the ignorance and suffering in the world, understands that in the larger scheme of things everything is proceeding exactly as it must toward the eventual goal of God-Realization, for each and every soul in creation. The former view has a desperate edge to it and carries with it an attachment to results, so that if results aren't obtained, one is tempted to resort to the use of force. The latter view serves others in the context of facilitating cosmic processes (in oneself and in others), tries not to be attached to the results, and in most circumstances is not tempted to resort to force.
This is a much deeper subject than reincarnation by itself, which I don't consider myself qualified to teach on despite over 40 years of intellectual study. You will find an in-depth treatment of this theme in Swami Vivekananda's work, "Karma Yoga," or in Meher Baba's "Discourses," both available on-line and listed in the Links section of this website.
The title, "Karma for Dummies," is meant to be purely fictitious, and not to refer to any book by that name which is published or yet to be published.
Music opening this page: "White Bird," by It's a Beautiful Day, from the album "It's a Beautiful Day"