He was a shepherd. It came time for both of them to bring a sacrifice to God to worship. Cain brought fruits and vegetables, and Able brought a first-born lamb. God looked at Abel's sacrifice and said that is good, and He looked at Cain's sacrifice and said that is not good. No, the message is very clear.
Fig leaves, fruits and vegetables, are easy.
Something has to give its life to fix things. In the days of Egypt when God told the people of Israel, on this night he said sacrifice the lamb, and take the blood of the lamb and whip it over the doorposts of every home. When I see the blood I will pass over. God said nothing about how wonderful you people are, or how energetic and how much you have achieved and how holy you are. He simply said this, when I see the blood I will pass over because something has to give its life.