Keith, just to be absolutely sure the site you're looking at takes the ACSI card, check on the ACSI card website (www.campingcard.com). ACSI is a Dutch Camping Organisation which 'inspects' campsites too - so not all sites bearing the ACSI logo take the card.
Basically if the site you want takes the ACSI card then Yes, you will get a discount on the camping fees. Site costs are in one of three price bands, 11 euros, 13 euros and 15 euros and that's all you pay for a pitch, car and caravan, two people, and electric hook-up. Normally you just show the ACSI card on arrival and pay at the end of your stay.
Normally you don't actually need to book ACSI sites - because the card is accepted in the low season you'll always find a space. We go earlier than you (last week of August) and have never had problems finding a pitch on any of the sites we've wanted.However you can book sites through the ACSI card website - there is a reservation form on the website which you can complete and email to the site if you'd rather make absolutely sure you've got a pitch. It does take confidence the first time you go without a booking, as we're so used to having to book sites months in advance in this country.
Don't worry at all about asking questions - it's the best way to find out what you want to know!