The Bible explains that there must be a sacrifice of blood before God can forgive sins:
If there is no sacrifice of blood, God will not forgive our sins. (Hebrews 9:22)
In the time before Jesus, people used animal sacrifices to do this:
A person must tell the priest if he has done any of these things. When he gives a sheep or a goat, the Lord will not be angry with him. The priest will kill the animal. (Leviticus 5:5-6)
But Jesus died and he was the ultimate sacrifice:
So Jesus died once as the sacrifice. By doing that, he took away all the sins of many people. He will come a second time. He will not have to die again for our sins. He comes the second time to rescue all who are waiting for him and make them whole. (Hebrews 9:28)
To read about why Jesus’ death means that no more animal sacrifices are needed, see the Book of Hebrews.