A negative power means the quantity is in the denominator of a fraction.
(2) to the -1 power means 1/2
6 to the -3 power means 1/(6 to the 3 power).
13xy to the -8 power means 1 / (13xy to the 8 power)
That's something like 1 / [ (13 to the 8 power) (x to the 8 power) (y to the 8 power) ]