Check with your employer to be sure, but it likely means TaXaBle SEVerance pay.
Employers use many different abbreviations and codes in Box 14..
Does this mean taxes have already been taken out or will I have to pay the taxes when I submit them?
The entry in Box 14 is included in your wages in BOX 1. Therefore, taxes have been accounted for in BOX 2.
As with all tax withholding it is an estimate. You may owe or get a refund based on other income and deductions.
Does that mean you are taxed twice? Should you still be taxed on all future severance payments as well?
No, you are not taxed twice for code TXB SEV in Box 14 of your W-2. Employers use Box 14 on Form W-2 to provide other information to employees. Generally, the amount in Box 14 is for informational purposes only. However, some employers use Box 14 to report amounts that should be entered on your tax returns. If you have any questions about the information in Box 14 contact your employer for an explanation.