It means there is only 1 vein in the cord instead of 2.

My daughter had this. She is completely healthy.

Its only a concern if theres are other markers for down's syndrome. If everything else is fine and they didnt suggest you to have a repeat scan then bub is probably fine.
My words exactly

DS2 had this as well, and is perfectly fine - he was even born prem at 36 weeks (not due to this though, due to me having placenta praevia) and although his birthweight was teeny (2.85kgs) - it was a great size for his gestation.

I too find it a little concerning that the Dr didn't know what it meant :O