Basically used to control current flow to your lamp. It all depends on your setup for the grow and what type of lighting you are using.
is this 400W HPS Sun System 10 Remote Ballast 120/240V: Express Light Bulbs just a ballast or a lamp as well?
The link you posted has reflector options right under the description if you need help picking out one to go with that ballast.
Um, what is needed is a mogul socket. To the OP, if you're going HID then you probably want to buy a light kit, that includes a bulb, socket, reflector, ballast, and wiring, ready to go out of the box.