Box 20

Yes, w/h generally stands for withholding. The money that an employer deducts from an employee's gross wages and pays directly to the government. 

Box 20 on your W-2 is for the name of the locality where you are paying the tax reported in box 19. It is likely saying that the city in Box 20 is the locality the amount reported in Box 19 was for.

If you have nothing in Box 19, you can leave it blank. If you have an amount in Box 19 and you can't tell the name of the locality, you should ask your employer for the locality name that they withheld tax for because you may have to file a tax return for that locality.