There are six faces in one die.

Therefore, the chance of any of them occuring once is 1/6

Now, suppose there are two dice with 6 faces each

Now there can be 36 ways of the outcome

E.g, (1,1) (1,2) ... (2,1) (2,2) (2,3) .... (6,5) (6,6)

Therefore the chance of getting two sixes is 1/36

You can also get it by multiplying (1/6)*(1/6)