The theoretical probability is the probability you'd expect before performing the experiment. So, assuming the die is fair, every outcome has the same probability. Since there are 6 numbers on the die, every number appears, theoretically, with probability 1/6.
The experimental probability is the probability you estimate after performing the experiment. You divide the number of cases a certain outcome happened, and divide by the total number of trials.
In this case, you performed 50 rolls, and the die landed on six 12 times. This means that the experimental probability is 12/50, or 6/25.
