In food, a sweetener is something added to food to make it sweeter, like sugar or a sugar substitute. In politics and business, a sweetener for a deal is something added to make a deal more tolerable to individuals who must approve it.
A related term is palatable. This is a fancy word for "something you can eat without suffering," so you add a sweetener to a deal to make it palatable (not easier to eat, but easier to approve). Often, we say more palatable here (indicating the deal is more acceptable, rather than less).
Of course, if we want to say less palatable, we do.
Example: President Barack Obama’s tax-cut deal likely will squeak through the Senate, according to congressional aides, propelled by a coalition of Republicans, moderate Democrats and members won over by last-minute tax sweeteners.