the salt would be to add some sodium and chloride to the water to 'replicate' the water used in Newcastle-Upon-Tyne, UK where Newcastle is brewed. i've never heard of using table salt to do this, but most LHBS will have brewing salts available. they're used when trying to match water from various brewing regions. i'd personally recommend against adding any brewing salt to water unless you KNOW the water you're using lacks the minerals you're adding. most tap water in the US is fine for brewing most beers, and unless your water is particularly soft or lacking trace elements, you could brew a Newcastle type beer without the addition of brewing salts.