This is a really paradoxical statement. In the physical world when you give something away, by definition, it no longer belongs to you. What once was your possession, now is the possession of someone else. In the spiritual world it is all turned upside down.
In God’s economy until you give your heart to Him, it belongs to the enemy. Before we find Christ we all think that our heart belongs to us and our thoughts are our own. This is not the truth. Your heart and mind either belong to the enemy, who keeps them and uses them, or your heart and mind belong to Christ, who gives them back to you.
This is the truth that is given to us in Galatians 5:1
For freedom Christ has set us free; stand firm therefore, and do not submit again to a yoke of slavery.
If you give your heart to Christ it is yours and it is free. If you keep your heart from Christ, it belongs to the enemy and you are a slave.
This is very basic and very serious stuff. I hope you have already given your heart away. If you still have questions about Christ or how to give your heart away I am second is a great place to find answers.