One of the most popular teachings of Buddhism is the idea of karma. This is the force that creates a cycle of suffering and reincarnation. Good deeds produces "seeds" with fructify good things either in this life or the life to come. Similarly, bad deeds do the same and produce bad things in this life or the life to come. In view of this, you can imagine, there is a call to live an ethical life.
More specifically, karma possess an idea of intentionality. Good intentions lead to good things and bad intentions lead to bad. This is fundamental principle of Buddhism. What is more, according to Thervanda Buddhism, there is no forgiveness or salvation from karma. It just is.
From one perspective, this idea of karma is impalpable. There are plenty of "bad" people that thrive and plenty of "good" people that suffer. The whole idea of karma seems like wishful thinking. They also seem to have to add the idea of reincarnation to escape the the fact that much of the world is unjust.