It means something changed from the first W-2 you received. You'll have to compare the two W-2's to figure out what changed. If the change occurred in Box 14 of the W-2 the chances are good that there's no income tax impact as that's pretty much a "memo" field. If dollar amounts in other boxes changed you should reenter the W-2 in your income tax return.
Of course if you've already e-filed then you'll have to amend your income tax return which you DON'T do until the IRS has completely processed your original "as filed" income tax return and you've received your refund or your check has been cashed.
This FAQ will guide you through the amending process:
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
Tom Young
