What is going on with this? i looked at each item in help but im not to smart, and to try to put them all togater gave me a headache

Work from inside out:

ROW(J$35)
gives: 35 (row number)

(ROW(J$35)+1-$AI20)
gives row number , adds one to it, then subtracts the value in cell AI20

ADDRESS((ROW(J$35)+1-$AI20),COLUMN(J$35),TRUE)
gives the address of the cell in column J

INDIRECT(ADDRESS((ROW(J$35)+1-$AI20),COLUMN(J$35),TRUE))
gives the value in the cell above

PRODUCT(INDIRECT(ADDRESS((ROW(J$35)+1-$AI20),COLUMN(J$35),TRUE)):J$35)
multiplies that value by the value in cell J35

PRODUCT(INDIRECT(ADDRESS((ROW(J$35)+1-$AI20),COLUMN(J$35),TRUE)):J$35)-1
subtracts 1 from that total.

So, for example if you have the following values in these cells:

J15: 2
AI20: 2
J35: 10
J34: 28

Your final answer would be 279.