While God revealed the 10 Commandments to Moses and the people of Israel, he revealed a "new" commandment through the ministry of Jesus.
The new commandment Jesus gave his apostles is in the Gospel of John.
This is technically not a new commandment, but is a further deepening of the original 10 Commandments.
According to the Ignatius Catholic Study Bible, the difference is in the type of love requested by Jesus.
It does not wipe out the old commandments, but simply helps us better understand how to live them out.
We are to follow Jesus' example and his new commandment, loving others as he loves us.