An explicit promise that reduces the buyer's risk. The best ones shift risk entirely to the seller.
Every purchase has risk: 'what if this doesn't work for me?' A guarantee is how you address that risk explicitly. A weak guarantee ('30-day return') barely moves the needle. A strong guarantee ('results or your money back, plus $500') tells the buyer you believe in the product enough to stake your own money on it.
The bolder the guarantee, the lower the buyer's perceived risk, the higher the conversion. People WAY over-weight risk. A strong guarantee often doesn't cost you much in refunds but boosts conversion meaningfully. Test this.