Here you can set the value of the currently edited tax for a particular country; please note that the country recognition is based on the Apply taxes based on setting (can be configured e.g. in CMS Desk -> E-commerce -> Configuration -> Store settings -> General tab -> Taxes section). The value can be either a percentage of the product price (by default) or a flat amount. If the tax is specified for both a country and a state, only the tax specified for a state is applied.
Further information about tax classes can be found in E-commerce Guide -> Managing your store -> Tax classes.