Why are you asking----I hope it is not because you are trying to use a pay stub to prepare your tax return. Do NOT use a pay stub to file a tax return. The information you enter into the boxes will NOT match the W-2 you receive from your employer. Employers have until February 1 to issue W-2's.
Box 3 shows the wages you earned that are subject to Social Security withholding. Box 4 shows how much Social Security was withheld from your paychecks.
"Social Security Tax Withheld" is 6.2% of "Social Security Wages".
"Social Security Wages" is generally your gross wages, and capped at $137,700 in 2020 for social security taxation. Tips reported to your employer get special handling on the W-2 but end up part of the wages taxed for social security.
If you're having pretax deductions for 401(k)s and the like taken out, these are included in your social security wages but not in your taxable (Box 1) income, so Box 3 (SS wages) is frequently higher than Box 1.
In addition, 1.45% of your social security wages, without cap, is withheld to help pay for Medicare.