Hello.
The voucher syou have been given will becatorgorised as either half board "standard" "plus" or "premium" depending on either what hotel you booked (if they were part of a deal) or what option you took at time of booking. First check which you have then look in either the latest brochure (from most high st travel agents) or on their website and you can get a list of restaurants you can use them in. There's quite a few choices, even for the cheapest ones and they include the option to eat in the hotels / parks / Disney village (the eating/shopping area).
You voucher is valid for either an all you can eat buffet and one drink or a 3 course meal from the set menu - dependant on what the service style of the restaurant you pick is. Anything extra you order can be paid for at the end of the meal.
The Rainforst cafe is expensive, they do not accept vouchers but it's a nice novelty time for the kids. Food isn't anything that special but made up for by your surroundings and nice service.
You can use some of the half board meal vouchers to pay for character dining. These are either at Cafe Mickey or Lucky Nugget Saloon. This is where you get your meal whilst characters come round, sit with the kids, pose for photos etc. They are alternated so you get to see a variety during your meal. Usually best to book this (book a time slot and they give you a reservation number for your table) as otherwise you may have a huge queue time waiting for a table, especially if you go during peak times.
Plaza Gardens restaurant is really pretty - it's just outside fantasyland in the main park and they do a lovely all you can eat buffet lunch or evening meal. Or the REndez-vous des stars (ratatouille themed) restaurant in the Disney studios park is lovely too - again an all you can eat buffet, byt only open until late afternoon. Both these places accept the half board meal vouchers.
Remember you get one voucher per person per night, not per day.
Hope this helps, any more ?'s just ask.