Two things: the dying-away of the old self, and the coming-to-life of the new.  (see Heidelberg Catechism Q&A 88)

• What do you mean “dying-away of the old self”?  – It is to be genuinely sorry for sin, to hate it more and more, and to run away from it.
• What is the coming-to-life of the new self?  – It is wholehearted joy in God through Christ and a delight to do every kind of good as God wants us to.