Mill rate is a tax rate — the amount of tax payable per dollar of the assessed value of a property.
Mill is derived from the Latin word millesimum, meaning thousandth.
As used in property tax, 1 mill is equal to $1 in property tax levied per $1,000 of a property's assessed value.
Government entities (Town of North Stonington in this case) set mill rates based on the total value of property within their jurisdiction, to provide the necessary tax revenue to cover projected expenses — roads, schools, emergency services, and so on — in their annual budgets.
Property taxes are calculated by multiplying the assessed, taxable property value by the mill rate and then dividing that sum by 1,000.