Cain and Abel were the first two sons of Adam and Eve. Cain is a farmer - a "tiller of the ground", and Abel is a shepherd. They brings gifts, offerings, to the Lord. Abel's gift is accepted, and Cain's is refused. Cain gets angry, and murders Abel.

Why would God accept one gift, and reject another? There's no apparent reason given in the story. Why would this story have been preserved, and included in sacred scripture?

In the Bible, these stories have an inner meaning:

"Cain" symbolizes faith that doesn't have love combined with it.

"Abel" stands for charity - a love of one's neighbor. This charity is the brother of faith; they are supposed to work together, shoulder to shoulder. When faith (Cain) isn't working alongside charity (Abel), it becomes harsh, and destructive.

Here's a very brief excerpt from Swedenborg's work, Arcana Coelestia, Section 366:

"Cain rose up against Abel his brother, and slew him" signifies that separated faith extinguished charity.

You can imagine how this might happen; a faith that is not driven by an underlying love of the neighbor could be harsh, and unforgiving, and could come to the point of trying to stamp out love if it is seen as a threat to "real" faith.

There's more to the story. It's very old, but still very relevant. You can follow the links, and explore the details.