"Keep thy heart with all diligence; for out of it are the issues of life." - Proverbs 4:12
When Heavenly Father speaks to us about our heart, He is asking for our entire life, the entire personality, character, body, mind, and emotions in the spirit of a person. The heart is the real person, not the person everybody sees.
The heart is the most important aspect of the spiritual body, and the heart attitude should be the major issue of every Latter-Day-Saint and Christian in the world. It is not lack of ability or potential that prevents most people from making progress and enjoying fulfillment in life. It is wrong heart attitudes that negatively affect our minds and thoughts.
There are many conditions of the heart. Some are positive, and some are negative.
"Amaziah was twenty and five years old when he began to reign, and he reigned twenty and nine years in Jerusalem. And his mother’s name was Jehoaddan of Jerusalem. And he did that which was right in the sight of the Lord, but not with a perfect heart." -- 2 Chornicles 25:1-2
King Amaziah is noted for having a negative condition of the heart. He did all the right things, but his heart was not right. Therefore, God was not pleased with him. That's a scary thing. We can do the right thing, and yet have it not acceptable to God because we do it with a wrong heart attitude.
Some of us accept callings out of duty, but dread the responsibilities and try to find ways to get out of it. Some of us may only pay tithes and offerings because it is a commandment, thus never experiencing the true fruits of obedience: gratitude for the things that we have. Some of us read our scriptures daily because we should, yet we haven't feasted upon the words to learn of Christ and to learn of Heavenly Father's will for us.
Doing the "right thing" is not enough. Doing the "right thing" with the right spirit is great!
Heavenly Father is more concerned about our hearts than He is about what we do, because if our hearts are right, what we do will eventually catch up with that. Who we are in our hearts is reflected in our thoughts and attitudes.