Probability concerns those things which are uncertain. A game of chance may bring this out more clearly. If you have a ball in one of your hands, you might make a wager with me that if I can guess where it is you’ll give me $2 and if I can’t I’ll give you $1. I have a 50-50 chance of guessing correctly. However, if the game changed somewhat and I had the ball and I was also the one guessing, you would not play the game, because I know where the ball is.
In the first scenario, given my knowledge, which is very limited, I have no reason to believe the ball is in either hand. This leads me to suppose the probability that it is in either hand is equal for both. However, in the second scenario, I know exactly where the ball is and there is no gamble or chance. In these scenarios, probability concerns my knowledge of where the ball is and nothing intrinsic to the scenario itself.