The term "washed in the blood of the Lamb" in the Bible refers to being washed by virtue of the blood of Jesus. His blood was spilled upon his death to cleanse the sins of humanity.
Jesus was referred to as a lamb as a way to indicate his innocence before his execution, going willingly to the slaughter to provide a means for humans to become blameless in the eyes of God. The blood sacrifice acts as a powerful advocate for self-sacrifice and mercy. According to the Bible, the cleansing effect of being washed in the blood makes the believer "white as snow."