Sacrifice means to sacrifice something for someone or something (e.g Jesus sacrificed his life for us.)
Sacrifice started when Adam and Eve ate from the forbidden tree; they disobeyed God. Adam and Eve now knew things only God should know so they knew that it was embarrassing to not wear clothes. God now felt sorry for them and killed a sheep to make them clothes, so from then on everytime there was a baby somewhere there was an animal killed for clothes for the baby to wear.
Jesus is the ultimate sacrifice because he gave his life for us, even though people didn't believe him and we were rude to Jesus; and Jesus still loved us.