what is effective tax rate on my turbo tax return cover page and how does it calculate?

Take a look at this article that explains the marginal and effective tax rates.   The highest tax rate you pay is typically referred to as your "marginal tax rate," but you usually only pay it on the amount of income in the highest bracket.

• Your "effective tax rate" is the average percentage of your taxable income that you owe in federal taxes.
• You can reduce your taxable income using your Standard Deduction or by itemizing deductions for state and local taxes or sales tax, charitable donations, casualty and theft losses, medical expenses, and mortgage interest and points.
• You can reduce the tax you owe on a dollar-for-dollar basis if you qualify for credits, including the Child and Dependent Care Credit, education tax credits, and adoption tax credit.