What it really does is almost identical to what you would do. It sends a keystroke (the same as if you'd have pressed it) that will cast the bobber. Then it will move the mouse (the same as if you'd moved the mouse) until the bobber is selected. Then it will wait and 'listen' to the speakers. When a fish bites, WoW makes the familiar 'splash' sound; at which point, the bot sends a click (again, exactly the same as you'd click).
The bot has a friendly pop-up that goes over the tiny amount of setup you'll need to do.
If you don't catch the fish - that's okay; the bot will simply keep fishing. There are always more fish in the sea. In just a few hours you can go from 0 to 400. And, included with your purchase is a fishing/cooking guide (I'm not going to lie to you - you can find these online, free, from Google). If you follow it, it will tell you the best places to go to get your cooking up too.