The backside of a chicken, while it is alive, is the one that virtually always faces the Sun.
There are two renditions of "backside", the one I just typed, which
is a one word noun. It means a person's buttocks.
The other is "back side", wherein, as you noted, back
is an adjective describing the noun back.
The back side of the chicken is placed against the foil and then, I guess
comes the next instruction.