If you are issuing a W-2 for an employee, you need to review the IRS instructions for the W-2 and the employers guid for fringe benefits.
https://www.irs.gov/forms-pubs/about-publication-15
https://www.irs.gov/forms-pubs/about-form-w-2
If you are trying to understand your own W-2, here are some basic facts.
Box 1 is taxable income from wages.
Box 3 is earned income subject to social security tax (FICA). Some kinds of income are earned but not taxable, like 401k deferrals. If you earned $100,000 but put $5000 into a 401k, your box 1 would be $95,000 and your box 3 would be $100,000. The maximum earnings subject to FICA is $162,000 for 2023, so box 3 should never be more than that.
Box 5 is earned income subject to Medicare tax. It is similar to box 3, except there is no upper limit. In addition, if you earn more than $200,000, you are subject to what's called additional medicare tax.
See here for more
https://www.irs.gov/taxtopics/tc751
When box 1 and box 3/5 are different, it is usually because some kinds of employer pre-tax benefits are subtracted from box 1, 3 and 5 but other pre-tax benefits are only subtracted from box 1.
There are also a few special classes of employees who can be exempt from social security or medicare tax and so their box 3 and box 5 might be zero or blank.
