Special offers give you the chance to buy coins.
In addition to standard rewards, you'll get bonus coins for each level you beat while the special offer is on. These coins will be kept in a special wallet, and you'll have the chance to buy them at a great price as soon as you collect the number listed in the offer terms. Bonus coins don't affect regular rewards for beating levels: you earn as many coins as usual with or without the wallet.
You can buy the coins as soon as you collect the required number, or you can continue saving to buy them later, in which case the deal will be even better. The number of bonus coins you get depends on the number of regular coins you receive for beating levels. The more regular coins you win, the more bonus coins you get.
You can't disable the wallet, but you can simply ignore it. You won't lose anything if you don't use it!