When line 9 on the Social Security Benefits Worksheet is zero or less, none of your Social Security benefits are taxable so it isn't necessary to fill in the other lines.
Your Social Security benefits may become taxable when you have other income. The $25,000 is a "base amount" for a single person. If one-half of your Social Security benefits plus your other income is greater than $25,000, then some of your SS benefits are taxable. For more information, please see Is my Social Security income taxable?
If you are still curious about the details, you'll find them in IRS Pub 915, Social Security and Equivalent Railroad Retirement Benefits.
