The concept of the sacrifice goes all the way back to Eden immediately after the Fall – Genesis 3:21. It established the principle that for the guilty to live, an innocent must die in his place. God clothed the guilty in the skins of the substitutes that suffered their penalty – a primitive picture of Christ’s righteousness being imparted to us.
But no animal could ever really be worth the soul of one who bore the image of God – Hebrews 10:4. It could only illustrate the promised Seed Who would one day bruise His heel while bruising the serpent’s head – Genesis 3:15 – the perfect Bearer of God’s image.
They passed this concept on to their sons, but only Abel took it to heart. He shed a substitute’s blood – his brother’s fruits suffered no death, no pain, no bloodshed. Cain’s offering did not demonstrate the horror of sin and its consequences, and it was not accepted.
So why was God pleased with such offerings?
First, they were object lessons of what the Messiah would one day accomplish, and offering them demonstrated faith in this coming promise.
Second, they were a fine, a penance, a reminder of the exceeding sinfulness of sin.
But God was only pleased with them insofar as they represented the faith and contrition that they were meant to show – Psalm 51:16-17.
I hope these thoughts will serve as a good foundation to get you started.