What is a no-fly zone?
It is a prohibition on all or certain types of aircraft flying through a designated airspace, over a country or region.
The zones require one or more parties to be willing to enforce them – that is, being ready and able to shoot down any aircraft violating the space.
They have been used in attempts to limit an ongoing conflict on the ground and to protect certain populations. But they are costly, requiring constant air patrolling and monitoring.
https://amp.theguardian.com/world/2022/mar/15/what-is-a-no-fly-zone-and-could-one-be-imposed-over-ukraine