It means the card wouldn't add anything you haven't already got, so you can simply press Skip. On a GEDCOM file card you'll see "Nothing new – everything here is already on" that person's record. On a People builder card it's "Nothing new in these boxes – the life story and pictures are checked when you merge", because Wikipedia's life story and pictures are only fetched when you merge. For example, say a file's card for Grandad George matches his record box for box – Nothing new, and Skip moves you on.
Where to find it
Settings (bottom row of the top bar, far right) → People builder
Only people with the "Add people" tick.
On a phone: if the Settings tabs are tucked away, press ☰ Menu (top left) first.
Still stuck? Talk to a real person: send us a message, or email support@mywholefamilytree.com