Atonement
Understanding how we are restored to a right relationship with God.
Ever since our rebellion in the Garden of Eden, we have been covered in sin. The way that we can get back into a right (righteous) standing with God is through Atonement or Kippur.
The most important holy day of the year for Jews around the time of Jesus was a day called Yom Kippur. On this day, the priests of the temple would sacfrice several animals for forgiveness on behalf of the whole nation of Israel.
Atonement or Kippur is crucial in Judaism for restoring the relationship between people and God, caused by sin (missing the mark).
Since the penalty for sin is death, God allowed His people to take an animal without blemish (highest quality, as close to perfect as possible) to be put to death in our place. This sacrificed animal absorbed our sin and washes the sin from us and covers us instead in righteousness and gives us a clean slate.
Besides Yom Kippur, a person would be required to make a sacrifice whenever they have sinned and there were other types of sacrifices that could be used like pigeons or grain and flour, depending on what a person may have done.
The idea behind these sacrifices is this: when you mess up, there is a penalty to be paid. God is a God of love but He is also a God of justice, and when the balance of justice is off then something needs to happen to restore the balance.