d66 means rolling two dice, reading the leftmost die as the "tens place" and reading the rightmost die as the "ones place."
Example: roll (5|2), read as 52 Greedy.
This is why the chart jumps the numerical sequence from 16 to 21, 26 to 31, etc. The probability of any given result on a d66 roll is equally 1-in-36 (2.778%).