In I and II Samuel we learn that the word heart is sometimes translated as mind, soul, desire, courage, or conscience. There are also several references to God's heart. 

I Sam. 2:35 " But I will raise up for Myself a faithful priest who will do according to what is in My heart and in My soul; and I will build him an enduring house and he will walk before My anointed always. "  And then in 13:14 " But now your ( Saul's) kingdom shall not endure.  The Lord has sought out for Himself a man after His own heart and the Lord has appointed him as ruler over His people, because you have not kept what the Lord commanded you." 
In response to God's covenant with David, he prays, ".... For the sake of Thy word and according to Thine own heart, Thou hast done all this greatness to let Thy servant know." 

Another very important reference to the heart is found in I Sam. 1:7 "But the Lord said to Samuel,'Do not look at his appearance or at the height of his stature , because I have rejected him; for God sees not as man sees, for man looks at the outward appearance, but the Lord looks at the heart."

God had given Saul a "changed heart" ( I Sam. 10:9)  But Saul was not a man after God's own heart. Saul did not keep the Lord's commandments.
David's heart was acceptable to the Lord.  His desires were God's desires . His conscience was sensitive to God's ways.  David sought to exult God and to serve Him with all his heart. He had a valiant and courageous heart because he inclined his heart towards the Lord. He was not perfect, but he did purpose in his heart to obey God's word.

Father, how amazing that You have revealed Your own heart to us!  Your desire, Your heart for us is that we would seek to do what is in Your heart, seek to have a heart like Yours, seek to know Your own heart.  O Lord, we desire to have hearts that are obedient, inclined towards You, that we might love and serve You with all our hearts and souls and minds.   Change our hearts and make them ever new.. growing in grace and truth and love. Fill us to the full with Your desires and thoughts and purposes by Your Spirit, through Jesus our Lord. For it is in His Name we come, amen.