The brothers Cain and Abel, Adam and Eve’s sons, offered their sacrifice to God: Cain (a farmer): field crops, God didn’t take it, and Abel (a shepherd): a firstborn lamb, God enjoyed it. Cain came  angry  about the refusal of his sacrifice and got violent although God had warned him not to. He went and killed his brother Abel in his rage.
God was merciful towards him and made a special mark on him (the mark of Cain) to protect him against counter violence and revenge so that nobody would kill him.

You must know:

In sacrifices for God blood had to be shed. Blood was considered to be the bearer of life, soul and vitality in the Old Testament.  In those days people wanted to reconcile with God by sacrificing animals. (In the New Testament / Christianity the faithful know that they are reconciled with God because of the blood of Jesus. We do not sacrifice anymore because Jesus died for our sins.)