Player Importance for your fantasy team tells you how important each player is in terms of the success of your team. The higher the importance rating, the better.
Generally speaking, any players with a "green" importance rating means that they are extremely critical to your team's success. Orange, is the second most important.
Player importance looks at your player's value right now, and is not neccessary predicting future performance. If you feel as though the player cannot keep up his production then it may make sense to trade him.
Player importance is also useful when viewing other teams in your league to see which player's are critical to your opponents success. They could be an injury away from being a below .500 fantasy team.