Special offer gives 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. The coins will be kept in a special safe, 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 the regular reward for beating levels: you earn as many stars as usual with the safe and without it.
You can buy all the coins at once as soon as you collect the required amount, or you can continue saving them up to buy them later, in which case the deal will be even better. The amount of bonus coins you get depends on the number of regular coins you get for beating levels. The more coins you win, the bigger the bonus. You get bonus coins for beating regular levels only.
You can't disable the safe, but you can simply ignore it. You won't lose anything if you don't use it!