To whet is to sharpen. You could whet a knife's blade with a whetting stone, or you could whet your appetite by having some Doritos.
The verb whet can mean "to stimulate or make more acute," and the word is often used in the phrase "whet [your] appetite," which can be used literally or figuratively. You could serve light appetizers to whet everyone's appetite for dinner or you could whet an actor's appetite by giving him a small role that inspires him for greater roles. You can whet other things in this sense as well — such as curiosity, fear, or pleasure.