A d66 is a type of dice pool that includes two separately identifiable d6s, in which the result is determined by taking the result of one d6 as the tens place of a number (i.e. the numeral on the face multiplied by ten) and the result of the other d6 as the units place. (This is similar to the way two d10s can be used to simulate a d100.) This gives 36 equally possible outcomes between 11 and 66, in which the only digits are 1, 2, 3, 4, 5 or 6.
Although d66 resembles dice notation, it is different from the standard interpretation of d66 in dice notation, which would be a single die in which all of the numbers from 1 to 66 are equally likely.
The result of a d66 roll is almost only ever used to consult on random tables, since the possible outcomes have significant gaps that would diminish its direct use in many other game mechanics. (That is, a d66 can produce a result of 16 or 21, but none of the numbers in between.)
Other similar dice pools include the d666 (which uses three d6s for 216 possible outcomes between 111 and 666) or the d88 (which uses two d8s instead of d6s), etc.