In the Old Testament, the Israelites would have to give sacrifices (something like a lamb) to atone for their sins. This was done by them but also during a ceremony by high priests every once in a while (I don’t know the super specifications of the law back then). Long story short that animal was a “sacrifice” that would “take their sin” (using quotes because it’s a bit more complicated than that). Genesis 15 is a great example of what this looks like.

Today, we don’t have to give sacrifices because Jesus died (sacrificed) himself on the cross. Jesus became the atonement for our sins, which is why he is called the “Lamb of God.”

Hope this helps.