Web, domains and email
TTL
Abbreviation for Time to Live
In short and in plain language
The TTL is a time limit in the Domain Name System.
It says how long an answer may be stored. This matters during a move. It decides how fast new records for a site and email work everywhere.
So it is usually lowered for a while before a move.
Definition
The TTL states how long an answer from the Domain Name System may be cached. During a migration it determines how quickly new records for website and email take effect everywhere. That is why it is usually lowered temporarily before a migration.