The prevailing tax rate is the sales tax rate that is applied by default to all of your products. To set the tax rate go to Products>Taxes> Advanced Tax Settings>Tax Groups>Prevailing Tax Group. Click the blue + Add Tax link. Fill out the Name, Tax Rate, From and To date, then click Save. Be sure to save your tax rate by clicking the Save button in the upper right hand corner. After entering and saving it there, it will appear here in Basic POS Settings. You can edit the prevailing tax rate under the Settings> Basic POS Settings> Prevailing Tax Rate page as well.
For more information on setting up prevailing tax rate and other tax rates, please click here.