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 tiller of the ground, Cain brought of the fruit of the ground (note: not all the fruit of the ground). The ground was cursed as a result of man’s disobedience and the fruit of the ground was the by-product of man’s effort and sweat (Genesis 3:17-19) which God did not accept as an acceptable offering.

Points to ponder:
God is not interested in our efforts and works for they cannot gain his acceptance. It is by grace (of Jesus Chris) and grace alone and not by our works that God accepts us in his beloved (Ephesians 1:6). Are you/Am I a tiller of the ground?

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