Aiigh, this is EXTREMELY hard to describe in words. but here goes:
BTW, this is a 4U topic, so if u just do 3U, you dont have to worry.
f'(x) = whatever
graph f(x).
Now, each point on the graph f'(x) is the gradient of f(x).
So when f'(x) = 0, then f(x) is having a turning point...