Atonement, in the Bible, is the act of making amends or reconciling [repairing] a broken relationship between God and humanity. It is the process of seeking forgiveness for sins [harmful or bad actions] through confession, repentance [expressing regret for your bad actions], and the offering of sacrifices [valuable things given up to God, such as food or jewelry]. For example, in the Old Testament, the Israelites would offer animal sacrifices to atone [ask for forgiveness] for their sins and restore their relationship with God.
Very Simple Definition:
In the Bible, atonement means making up with God when we do something wrong. It is like saying sorry and trying to fix our relationship with Him. For example, in the past, people would give important animals to show they were sorry and wanted to be forgiven by God.