If the prices of the products already includes an added tax to its price already, then you would want to set the “Tax Inclusive” feature to ON.

Example - Tax Inclusive (ON)
Product: Headphone - $100.00
Tax Rate: 10%
Invoice subtotal: $90.91
Tax amount: $9.09
Invoice Total: $100.00

Example - Tax Inclusive (OFF)
Product: Headphone - $100.00
Tax Rate: 10%
Invoice subtotal: $100.00
Tax amount: $10.00
Invoice Total: $110.00