For the experiment of rolling a fair dice, each face has an equal probability of showing up.
Let 'p' be the probability of getiing a particular face.
So the probability of rolling any of the 6 face is 'p'.
Now, the sum of all probability should be unity,
Thus, the probability of any face is 1/6.
Hence, it concludes that the theoritical probability of rolling a 2 is equal to 1/6.