The probability is 1/36.
The outcome of the first roll does not affect the outcome of the second roll, and so the probability of rolling any number on a fair six-sided die remains 1/6 for both rolls. This means that the probability of rolling a 5 the first time and a 6 the second time are both 1/6, and to find the probability of 2 things happening we have to multiply, so we end up with 1/6 × 1/6 = 1/36
I hope this helps! Let me know if you have any questions :)