The warning means you are about to move to another copy of the outdoor world.
- Refresh Now starts the change early. Get out of combat and somewhere safe first.
- Okay only hides the large notice. The countdown continues.
- After a refresh, check that grouped friends can still see each other.
- It is a normal game notice, and nothing needs reinstalling.
Unconfirmed whether every party member gets the same timer.
Source Wowhead's recap of the phasing notice, 19 Sep 2026.