The 5a amount is the gross SS you received (before any medicare premiums or other deductions)...the 5b amount is the sub-amount that is considered taxable income.
The IRS will consider up-to 85% of your SS income to be included in your taxable income, when your other income (including tax-exempt dividends&interest), plus 1/2 your social security, reaches the following levels:
1) Married Filing Jointly: $32,000
2) Single or Head of Household: $25,000
3) Married Filing Separately: 0
TTX software will calculate and change the box 5b value as you continue to add in your various sources of income.
2019 1040 line 5b is the taxable part of Social Security.
Up to 85% of Social Security becomes taxable when all your other income plus 1/2 your social security, reaches:
Married Filing Jointly: $32,000
Single or head of household: $25,000
Married Filing Separately: 0
To see the Social Security Benefits Calculation Worksheet in Turbo Tax Online version you would have to save your return with all the worksheets to your computer. Or if you are using the Desktop program you can switch to Forms Mode (click Forms in the upper right) and click on SS in the list on the left side.