Jesus' death on the cross is an act of atonement. The Fall, when Adam and Eve ate the forbidden fruit, destroyed the perfect relationship between humans and God and introduced sin into the world. Some Christians believe that because of this all people are born with sin - 'original sin'.
The sins people commit daily, take them further away from God.
God sent Jesus to earth to repair the broken relationship between God and people. Through Jesus' death, He paid the price (atoned) of all our sin. This was possible because He did no sin.
This offers humans salvation, which means they can live on for eternity with God in heaven.
Before Jesus, the Jews would often use animal sacrifices to 'atone' or make up for the sins they committed. This was a temporary measure.
The sacrifice Jesus made by dying on the cross meant He became the ultimate sacrifice to atone for the sins of all humanity. Jesus became a sacrificial lamb to take upon himself the sins of the world. There was now no longer any need for animal sacrifices and everyone can know God - not just a special group of people (the Israelites who became known as Jews).