Cain and Abel

What does the saying 'the mark of caine' mean?

In the book of Genesis in the Bible, Cain killed his brother Abel. God cursed Cain and set a mark upon him. See Genesis 4:9-13.

Also the source of "Am I my brother's keeper?"

The phrase can mean bearing a burden of guilt, but there are others including use in racial discrimination - e.g. some believe(d) that the Mark of Cain was black skin.