If you consider the tip of the dart a point, then your odds are 0. You can't hit a point with another point.
However, when I throw a dart into a dartboard, it leaves a roughly 1 mm in diameter hole in the board. If you select a point on the board, then it is possible I hit that point because it is possible that the point lies within the 1 mm range I've created.
For example, suppose a computer selected a random number between 1 and 2. Because there are an infinite amount of numbers between 1 and 2, the odds of me guessing it correctly are zero, no matter how many guesses you gave me. But since the range between 1 and 2 is finite, then if I gave a range such as "the number is between 1.1 and 1.2" there would be a 10% chance I was correct.