What is mean "under lockdown"?

Not sure. Lockdown usually refers to prison when all prisoners are confined to their cells to prevent violence. Manhattan has a very large population to be under lockdown. I almost expect curfew to be a better word choice.

yes you right thank you so much.

But I need more Explain because still It's not my answer.I mean need more clearly answer..

I can't be of much more help. It would take a great deal of violence to lock down (impose curfew) on Manhattan, not to mention Brooklyn.

You're welcome, by the way. Wish I could be more helpful.

this sentence is here please look maybe can help.
https://www.youtube.com/watch?v=7oXSfUzjjV8

I got "Video not Available", so probably not available in the US. I did get the name Tom Clancy, so I assume it comes from a fictional novel. Yes, Clancy could have an emergency so great as to lock down two boroughs of New York City.

For what it's worth, Tom Clancy died some years ago and there is at least one person allowed to use his name.

Okay you right but now I try to translate this video and it's worth for me cuz I need to know the transcript.

No one allowed in or out of the city