The Book of Leviticus is about the law of God. The law teaches us that God is holy. The Book of Leviticus contains many rules for the Hebrew people. But we do not obey all the rules in Leviticus today, because we trust Jesus (see Galatians 3:23-25).
The Book of Leviticus also describes the duties of priests during the time of the Old Testament. And, in Leviticus 23, there is a list of special holidays, when the Hebrew people praised God together.
God also told the people that they must kill animals. And, they gave these animals to God. The people did this so that God would forgive their evil deeds (Leviticus chapters 1-7). This teaches us why Jesus had to die. Jesus died so that today, God forgives our evil deeds. We must confess our evil deeds to God. And we must invite God into our lives.
The Book of Leviticus is in the Old Testament part of the Bible.