These used to confuse me, too.
So, just like f(3) means that we replace x with 3 in our equation, f°g(x), aka f(g(x)), means that we replace x with g(x).
Since f(g(x)) = (x - 8)4 and f(x) = x4, you'll notice the difference is that x has been replaced with x - 8, so g(x) = x - 8.
Hope this helps!