When rolling two six-sided dice, the theoretical probability of getting a sum of 7 is 6/36 = 1/6 since there are 6 ways to roll a 7 (see the list of sums below) out of 6*6 = 36 outcomes total.
The experimental probability is just whatever results you record when you roll the two dice yourself. This will vary person to person. At first, the experimental probability is likely not going to be close to what it theoretically should be. However, as the number of rolls increases, the experimental probability should slowly approach the theoretical value.