OK, assuming that you have a gameshark for the PS2 the codes enable you to get lets, say, 10,000,000,000,000 credits! Or they can be used for changing parts between vehicles. For example: you can put a Viper engine into a Mini if you wanted.
As far as entering them, you need to go to "add new code" A screen will appear and this is where you will name your code, (i.e. Viper Engine.) you will then go to the next screen where you will enter the code that you have.
It should read very similar to this.
1c041378 858fa819
after the code is entered it will be saved automatically on your gameshark. When you want to use that code just check the box that is next to it and press the start button.
"M" (I don't know what it stands for) is the "enable codes" code. It must be on to have any code that you have selected to be applied in GT3. If it is a part code it will be applied to the 1st car in your garage.
I hope this helps you out. There are many threads that can help you out.
check out
http://63.169.100.88/ubb/