DNS
Also called Domain Name System
The internet's address book. It turns a name like shop.example.com into the numeric address of a computer. Answers get saved along the way, and each one says how long it may be kept, which is called its TTL.
Agents can edit a DNS record in seconds and then tell you it's done, while people keep arriving at the old server. The waiting is decided before the change, by the TTL already out there, so a move needs planning days ahead.
Say it to your agent
“We're moving servers on Friday. Today, lower the TTL on the A record for shop.example.com to 300. On Friday, change the record, keep the old server running for at least the old TTL, and show me dig shop.example.com +noall +answer before and after.”