What does the priest express when he presents this sacrifice?
Leviticus 4:4 (ESV)
4He shall bring the bull to the entrance of the tent of meeting before the LORD and lay his hand on the head of the bull and kill the bull before the LORD.
By performing this rite the priest was acknowledging his offence before the Lord. This was an expression of repentance.
4 He shall bring the bull to the entrance of the tent of meeting before the LORD and lay his hand on the head of the bull and kill the bull before the LORD.