Assuming they are 6 sided die. There are 5 numbers less than 6 on each one.
Probability one die is less than 6 would be 5/6
Probability the second die would be less than 6 would also be 5/6
The probability they are both less than 6 would be found by multiplying each probability together:
5/6 x 5/6 = 25/36