The word heart in Pro 4:23 refers to the core of man. The core of a man is his spirit. Afterall man is a spirit who has a soul and lives in a body. According to Pro 4:23 you must diligently watch what you expose your spirit to. Your spirit is your production center - Luk 6:45. This is why it is so important to guard your spirit. A good man produces good things out of the good treasure in his heart. Likewise an evil man produces evil things out of the evil treasure in his heart.
Your eyes, ears and mouth are the doorways to your spirit. Whatever you allow to enter in through these doorways will end up in your mind. Eventually they will transit from your mind to your spirit. Therefore to guard your heart you must be selective about what you watch, listen to and say.
If you want to produce good things then you must stay committed to protecting your spirit. You must stay in the presence of God by worshipping, praying and fellowshipping with the Word of God. Maintain the right environment so that you can produce good things.