This formula is just highlighting the Even rows in the Pivot table:
Mod(RowNo(), 2) just means Modulo(RowNo/2). So when we are on RowNo 1 the remainder will be 1 because Modulo(1/2) = 1, where as for every even number remainder will be 0.
So this formula =If(Mod(RowNo(), 2) = 0, RGB(159,159,159)) is just highlighting the even rows in the color grey.