The latter part of Genesis 4:2 speaks about the professions of the sons of Adam. Abel was a keeper of sheep, and Cain was a tiller of the ground.

As keeper of sheep, Abel brought of the firstlings of his flock and of the fat thereof and the Lord accepted his offering. Leviticus chapter 3 speaks about the procedures of a peace offering and highlights that the fat of the animal being offered is a special gift to the Lord (Leviticus 3:9) and that all of the fat must be presented to the Lord (Leviticus 3:9,14) because all of the fat is the Lord’s (Leviticus 3:16). In other words, not only did Abel brings the first of his flock, expressing that God was his priority, but he also brought a complete offering (all of it including its fat), expressing that God was also his pre-eminence.

Points to ponder:
God wants us to offer ourselves as a first and full offering – a living sacrifice (Romans 12:1). He was us to give him priority and pre-eminence.

On a separate note, Jesus is the chief Shepherd (1 Peter 5:4), and the people are his sheep (Luke 15). Jesus asked Peter, if Peter loved him and told him that if Peter loved Jesus, Peter must keep i.e., feed and take care of his lambs/sheep (John 21:15-17). In like manner we are commanded to love God totally (Mark 12:30) and we are commissioned to be keepers of his sheep (John 21:15-17). Let us be keepers of sheep, which is an expression of our love of God. Are you/Am I a keeper of sheep?

Genesis 4:2-5 (KJV)
2 And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
3 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the Lord.
4 And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the Lord had respect unto Abel and to his offering:
5 But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.