Like Gam3r says, everything in the book is necessary knowledge for a basic SCUBA course--but it will likely all become clearer and less abstract when you're actually doing the practical stuff.
All the information about how SCUBA gear functions is to explain how to assemble it, use it, and take care of it--so that it carries on working properly all the way through the dive. Once you're actually playing with the gear for real, it becomes a lot easier to understand.
All the information regarding wind, waves, currents and temperatures is to inform you of the various factors of which you need to be aware when making open-water dives. Again, this abstract information will make more sense once you've experienced these for real, when your instructor helps you plan and carry out your open-water training dives.
Likewise, the book discusses in detail many of the possible problems/emergencies a diver might face underwater (e.g. mask or mouthpiece loss, or getting a cramp, or needing a rescue, or running out of air, or malfunctioning SCUBA gear). Many of the basic diveskills taught on an entry-level dive course are actually about how to deal with such emergencies; although a careful diver would hardly ever need to use any of these skills for real, being able to do so 'in the unlikely event of an emergency' might well make a vital difference to his survival chances.
All the 'science-y' stuff about pressure/volume relationships is basically to explain why and how you need to equalise your ears and mask, why you should breathe continuously and never hold your breath while SCUBA diving, and why your cylinder contents get used up quicker at deeper depths. Once you've actually started experiencing the effects of pressure changes underwater (e.g. ear popping), the abstract theory becomes much more understandable.
Probably the _most_ abstract part of a basic dive course is the stuff about dive planning and nitrogen absorption under pressure. Although this is also primarily related to pressure and pressure changes, if you haven't yet fully grasped the more basic concepts, you'll just have to take this on faith for now, since you can't actually feel it happening. That said, you CAN see the effects of sudden depressurisation on dissolved gas by e.g. cracking open a soft-drink bottle, but in this case it's better NOT to experience this (i.e. decompression sickness, 'the bends') yourself for real!
Finally, I would point out that you're not 'going to get certified' this weekend, you're going to go on a dive course. Getting certified (or not) depends on you--it will only happen if you learn to do everything you need to be able to do to pass the course. This is relatively easy, provided you put some effort in.
So the best advice I can give you right now is, read as much of the book as you can (preferably all of it), do any quizzes it contains, make a note of anything you're not sure about, pay close attention to your instructor during the course, both in the pool and the classroom, and ASK QUESTIONS about anything you still don't understand. Oh, and most important of all, try and have fun doing it--that's the whole point of diving, after all.
If you have any specific questions about diving, feel free to email me through Y!A. All the best.