SALT in shorthand for state and local tax. While you'll still be able to deduct your property tax, state income tax, and other local taxes on your 2018 tax return, this SALT deduction will be limited to $10,000.
For more information about the changes coming in 2018, see Tax Reform Legislation: What You Should Know.
For the state taxes which amount contributes to the total counted for SALT? Is it the withheld amount for state taxes or my actual state tax paid after my state refund? If it is the amount withheld am I being penalized for having too much withheld? If it is my actual state tax, how will that be calculated since that is usually determined after my federal tax return has been prepared?