Delicious; tasty enough to make you salivate.
The term mouth-watering has been used since the late 18th century to describe people whose mouth was salivating at the thought of food. The earliest citation that I can find of it describing appetizing food itself is from the mid 19th century, in the US fine arts journal The Southern Literary Messenger, 1847:
"He talks in an unctious mouth-watering way of British oysters and Falernian wine."
Mars, the manufacturers of the fruit sweets Opal Fruits alluded to the phrase in their advertising jingle for the product 'Opal Fruits: made to make your mouth water'. The sweets are sold in the USA as Starburst and, since 1998, they were renamed as that in their original market of the UK and Ireland