This means that if you're experienced in some topic, you can make a judgment about it. If you're good at guessing the speed of a car, you will be able to judge its speed relatively good. However, in order to have experience on what is good and what isn't you first need to have bad guesses - bad judgements. Only once you have done some bad judgements will you have enough experience to judge the correct speed of a car.
This saying of course applies to other aspects of life as well :)
