It is dividing the value in cell AD11 by the value in cell Z20.
The $'s around the Z20 mean that if you drag or copy that formula to another
cell, the reference of AD11 will change but Z20 will remain (an absolute
reference).
It means that the cell where the formula resides will show the result of
dividing the value of the cell AD11 by the value of the cell Z20.
The $ signs in the second reference mean that if the formula is copied to
other cells, Z20 will remain Z20; it will not adjust to the relative
position of the cell where it is copied to, as the AD11 part will.