According to the Bible, God disregarded the offering made by Cain because it was not given with the right heart and attitude. In the book of Genesis, it is written that Cain was a farmer, and he brought some of the produce from his fields as an offering to God. His brother Abel was a shepherd, and he also brought an offering to God of the firstborn of his flock.
God accepted Abel's offering, but he did not accept Cain's. The Bible does not explicitly say why God rejected Cain's offering, but it is generally believed that Cain's offering was not made in faith and obedience to God. Cain's heart was not in the right place when he made his offering, and he may have been jealous of his brother Abel's acceptance.
The story of Cain and Abel is often interpreted as a lesson about the importance of giving to God with a humble and obedient heart. God does not just look at the outward appearance of our offerings, but he also looks at the attitude and motivations behind them.