Việt Kiều literally means something like "Vietnamese sojourner" or "overseas Vietnamese," and it is the term most commonly used inside Vietnam to describe Vietnamese people living abroad and their descendants.
The word carries different weight depending on who is using it. In Vietnam it is often simply descriptive, though it can sometimes imply wealth, foreignness, or being a guest rather than a local. Abroad, some people use it comfortably about themselves; others find it distancing, since it can suggest they are no longer fully Vietnamese. Younger Vietnamese Americans often prefer "Vietnamese American" or simply "Vietnamese."
Being marked as Việt Kiều can be an odd experience to encounter for the first time — being visibly Vietnamese, and immediately identified as not-from-here by accent, clothing, posture, or the way you hold money.
None of that makes anyone less Vietnamese. It does describe something real: growing up in the diaspora produces a different Vietnamese experience, not a lesser one.