The essence of Psalm 23 is to trust in God to lead the way, even through life's trying times. Psalm 23 brings comfort to those who have experienced troubles.
Psalm 23 is also titled "A Psalm of David." David was a shepherd, and he understood how deeply a shepherd cares for his sheep by both protecting them and providing for them. By making the sheep "lie down in green pastures" and leading them "beside quiet waters," the sheep have a chance to rest and be restored. Just as the shepherd knows the paths to lead the sheep to green pastures, God knows the path to lead his people to eternal life.
The phrase "shadow of death" found in the fourth verse can also be translated as "deep darkness" and symbolizes extreme danger. As the shepherd leads the sheep to green pastures, the sheep may have to cross deep ravines or valleys. As God's people travel through life, they may encounter difficult situations. By trusting the shepherd, the sheep travel safely. By trusting God, his people arrive safely to their eternal reward.
Psalm 23 concludes with David rejoicing that he "will dwell in the house of the Lord forever." Just as the sheep enjoy the abundance at the end of the trail from following the shepherd, God's people find "goodness and love" and enjoy abundance in God's house by allowing God to lead them through their darkest hours.