Web, domains and email
TTL
Abbreviation for Time to Live
In short and in plain language
The TTL says how long an answer about a domain may be kept in a cache.
When a domain moves, it decides how fast new records work everywhere. This is true for the site and for email.
So it is usually set lower for a while before a move.
Definition
The TTL states how long an answer from the Domain Name System may be cached. During a domain 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.