It is the last day of the High Holy Days and the last day of teshuva (a 40 day period when people repent for sin). The word atonement means "to cover". On the Day of Atonement, the sins of the Jewish nation were covered by the blood of the sacrifice. The procedure is detailed in Leviticus 16. On this day the High Priest entered the Holy of Holies four times and he:
This is the only day in the Bible that required a day of fasting (Leviticus 23: 27,32; Jeremiah 36:6). Jesus fulfilled the Day of Atonement when He went into the heavenly Holy of Holies with his own blood and He fulfilled it with his own blood for the sins of the world. On this day:
We receive forgiveness and Sozo (being saved, healed, delivered) when we change our minds, perceptions, and attitudes aka repent. When we do this we a purified before God so that we can have continual fellowship with him.
1 John 1:6-9
6 If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth:
7 But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin.
8 If we say that we have no sin, we deceive ourselves, and the truth is not in us.
9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.