Jesus' death on the cross paid the price that Man was to pay for sin.  As God had spoken back in Genesis and pronounced the penalty of disobedience (eating from the tree of knowledge of good and evil)to be death, there was a debt to be paid.  Because all of humanity entered into sin for this act, Man had to pay the price.  But only God had the power to satisfy the debt of the entire human race.  Thus Jesus, the second person of the Trinity, fully God, fully man paid the ultimate penalty.

Remember that Jesus is Fully God...Fully Man.  That helps to understand how He was able to pay the price for our sins.   What an incredible sacrifice!!