Chapter Seven tells us that each of the tributes will have three days to practise and train in a range of skills relating to weapons and survival before having a chance to demonstrate what they know in front of the Gamemakers. Each tribute will have fifteen minutes to show what they can do. However, if you are after the true significance and meaning of the ratings, you need to look at the beginning of Chapter Eight, which tells us how important the ratings are:
It gives teh audience a starting place for the betting that will continue throughout the Games. The number, which is between one and twelve, one being irredeemably bad and twelve being unattainably high, signifies the promise of the tribute. The mark is not a guarantee of which person will win. It's only an indication of the potential a tribute showed in training.
The higher the mark, the easier it is for various tributes to gain sponsorship, which can make the difference between victory and miserable death in the actual arena. The tribute ranking system is therefore a method of helping the betting but also it is a very important opportunity for tributes to attract sponsors who will support them by sending them specific goods that they need.