A tax shield is the reduction in income taxes that results from taking an allowable deduction from taxable income.
[1] Since a tax shield is a way to save cash flows, it increases the value of the business, and it is an important aspect of business valuation.
The concept was originally added to the methodology proposed by Franco Modigliani and Merton Miller for the calculation of the weighted average cost of capital of a corporation.
The reason that he was able to earn additional income is because the cost of debt (i.e. 8% interest rate) is less than the return earned on the investment (i.e. 10%).
The 2% difference makes income of $80 and another $100 is made by the return on equity capital.