The specific thing the prospect gets, at what price, with what guarantees. The core of the sale.
Marketing people obsess over copy and ads. They should obsess over the offer. The offer is what you're actually giving the customer: the product, the price, the bonuses, the payment terms, the guarantee. A great offer can survive mediocre copy. Great copy can't save a weak offer. Before you tune anything else, tune the offer.
Most businesses 'have' an offer, but they've never actually engineered it. Spending a day just on your offer (what's included, what's bonus, what's the guarantee, what's the term) often moves conversion more than six months of copy tweaks.